Buying second-hand looms can be a great way to get your foot in the door with a very, very satisfying hobby, even a profitable business.  But, remember, as in all purchases over the internet, you need to be an informed buyer.  In the case of weaving equipment, sellers do not always understand what they have, especially when they are offering items from an estate sale, and, in all sincerity, might not describe the item accurately, so you need to be informed.  If you can manage it, try and take a beginning weaving class before you go shopping for your own loom.  You will have a much better idea of what is comfortable for you and what you might like to "grow into" down the line.  I often tell people to go ahead and get a little bit more of a loom than you think you want now, since, in a year or so, you will want that "more."


Firstly, a little terminology.
A LOOM is the tool upon which you WEAVE.  As a verb, "loom" means to "hover over."
HARNESSES are the frames that hold the HEDDLES.  They move up and down and create the SHED.  You will often see them referred to as "SHAFTS," but more commonly they are just called HARNESSES.
The CASTLE holds the HARNESSES/SHAFTS and usually forms part of the support for the frame of the loom.  If they are enclosed, with the harnesses either hanging inside them or rising up inside them,  and you can put things on top of the loom, it is a "HIGH CASTLE."   If the harnesses rise up inside or even above the castle, it is a "LOW-CASTLE."
The HEDDLES are the long, thin thread or metal  items inside the HARNESS/SHAFT  that hold each individual thread of the WARP, or lengthwise threads.
The SHED is the opening created by lifting the  warp threads  in different combinations.
SHUTTLES are the sticks or "boat-shaped" tools used to pass the WEFT threads across the warp.  A BOAT SHUTTLE requires a bobbin that has to be filled and re-filled.
TREADLES are foot-operated and raise the HARNESSES.   You make the patterns in weaving by raising the harnesses in different sequences.  You can tie one treadle to one harness each or, on most floor looms, you can tie more than one harness to each treadle so that creating a pattern is a little easier.  If you want to change the pattern, you need to re-tie the treadles/harnesses.   
WARPING BOARD is a square or reel-type tool with pegs in it for making the lengthwise threads that will be the WARP on the loom.
The WARP is the lengthwise threads, kept under tension, upon which you weave.  The crosswise threads are the WEFT, WOOF or FILL.  More commonly-seen is "WEFT."
The WARP BEAM is the round beam at the back of the loom upon which the WARP is wound.  A comparable beam on the front is the CLOTH beam (guess why!).  You will find the WARP threads attach to these beams with either a cord (lashing cords), a metal dowel or a canvas "apron" or a combination of those things.  If a loom has the cords or the apron, look carefully at the pictures for signs of wear and damage, if you can't tell ASK THE SELLER to either write an assessment or take more pictures.
A REED is a metal frame containing strips of metal at set distances apart and determine the number of threads per inch your warp has: 8, 10, 12, 15 are the most popular.  These are prone to rusting but can be easily cleaned.
The BRAKE is there to keep the beams from turning until you want to advance your warp.  It will either be a ratchet type (big, toothy "gear') or friction which works with a cable that tightens and loosens.
Lastly, some looms work with a system of "LAMMS" which are attached to the harness cables.  On this kind of system, your treadles get tied up to the LAMMS.  You will find this on, for example, a Macomber loom as well as looms from all the "majors."
Lastly, there are some specialized systems you might see: DOBBY looms have a mechanical or computer-driven system to lift the harnesses in a programmed sequence and you only have one or two treadles.  These can have up to 40 harnesses.  A JACQUARD loom has a mechanism that controls each thread individually and is used for weaving "pictures" and other complex patterns.  I have yet to see one of these on ebay, and if you are in the market for one of THOSE, you don't need to be reading this!

Buying a floor loom:
1.) How much space do you have for the loom?  Aside from the "footprint," a loom needs a good deal of space around it to accomodate benches, warping equipment (more on this later) and lighting.  So, you need to add perhaps  3 feet on every side for your weaving space.  
If you have the space for it, a floor loom in a good choice.  It's also good if the loom can partially fold up when not in use.
2)  How much do you have to spend on equipping yourself to weave?  The loom is just the first of several things you will need.  You need a warping board  or sectional-beaming equipment.  YOu will need shuttles and, depending on what kind of shuttle you use, you will need bobbins and a winding tool for the bobbins.  Oh, yeah..you will need yarn.
3)  Check with the buyer if they are offering shipping.  Are they planning to disassemble the loom?  Are they sending you instructions for putting it back together?   Does the manufacturer still offer customer service, parts?   If you need to go pick it up, you need to find out what the dimensions are to make sure that it will fit the vehicle you are bringing.
4) Look carefully at the photos.  Do you see much rust?  Bent harnesses? Anything look uneven?  These are all danger signals.
5) Now, here's something you might not think of right off the bat: how tall are you and how far can you reach?  It's not much fun to have to keep adjusting if the loom is not at the right height relative to yours.  Granted, this is not very often a consideration, but it can be.
6)  You might also ask how big the shed opening is.  Most floor looms have a good, wide one, but you never know.  It needs to be wide enough to easily pass the shuttle through the warp.
7)  How many treadles does it have?  If you have one treadle per harness, it will be a little harder treadling a pattern that, for instance, requires you to raise or lower three harnesses at one.  If you have more treadles than harnesses, then, for example, you can have some dedicated to your plain weave, then the rest tied up for your pattern.
8)  Is the brake working smoothly?  This is one of those times to "ask the seller a question."
As to types of floor looms, here is an excellent explanation of the different types of loom mechanisms:
http://www.leclerclooms.com/cont.htm

Table looms:
A nice way to get your feet wet is a table loom.  As the description says, this is a smaller loom that will sit on a table.  Instead of treadles, the table loom has levers on the top, front or sides for raising the harnesses.
Table looms generally "max out" at 22 inches, although there are a few that are wider.  They usually start at 15" but one or two will have 8" weaving widths.  
1)The smallest is a pressed-tin loom, usually with four harnesses, but sometimes more,  made by the "STRUCTO" toy company starting about 1910.    This company made the looms until about 1972, when the patent was sold to Dick Blick art supplies.  They stopped production in 1978.  Originally meant as a toy, it is still a great little working loom.  That said, you will need to watch the pictures VERY carefully for amount of rust etc.  Be especially on the look-out for bent frames or other parts.  You can find these with anything from 2 to 16 harnesses!  One VERY important thing to notice about these is the warp beam.  Many of these have a warp beam that is only usable with pre-wound spools that are no longer available.  While they can be refilled, it's a pain in the neck to do so.  Sometimes you will see them offered with both a "regular" wooden warp beam as well as the metal spools-only beam.    The pros on these are that they are light-weight, & very small and nice ones are very collectible.  Cons?  Well, they ARE prone to rust.  They are uncomfortable to warp and the brakes often slip.  Sometimes the tops of the harnesses break off and you have to solder them back on.  It is also very hard to find extra hedddles to fit these..  You can certainly use the ""Tex-solv" heddles on these as you can on any loom since they can be found in a length that fits, (the 6 1/2" size) albeit a little tightly and not all suppliers carry that size.

Wooden, multiple-harness table looms typically start at 15" although one manufacturer is making an 8" version based on the Structo.   Some older table looms only have two harnesses, but, as long as it is a company like Leclerc, you can order a completely new castle for it with four harnesses.  In fact, most table looms will take one or two more castles of 4 harnesses each.   Check with the manufacturer, though, before you buy to make sure parts and modifications are still available for the model you are looking at, or ask the seller about that.   Again, check, or ask the seller if the frame of the loom is warped or  any of the wooden parts are split.  The harnesses are usually metal, so you want to make sure these are not bent.
There are a lot of good reasons to have one of these:  you can easily take them to classes and workshops or on vacation.  (Although I would not recommend folding them up and checking them as luggage if you're flying.)  They are comfortable to warp and you can find a table that puts them at a comfortable height.  If you have mobility issues or are "vertically challenged" as a  Little People friend once put it, the option to place it high up or low down is nice.   Cons?  Well, most of them will not carry more than 5 or 6 yards of warp at any time, although I once put 10 yards on one.   Also, the narrow width will restrict some of you end-product ideas unless you learn to double-weave.
Rigid-heddle looms are simple frame looms with a plastic "slot and eye" frame through which you pass the warp threads.  Some have two heddles, which offers you the option of doing 4H patterns, although getting a variety of patterns out of them will present a challenge to your ingenuity.  Pros?  Of course, the portability is nice, nice, nice.  They tend to be far less expensive than the Multi-harness table looms and the limitations sometimes are nice since they make you concentrate on colour interactions.  Cons?  see above about weaving width and you will have to work at getting different weaving patterns out of them, although it is completely possible to do quite a variety with just the plain-weave that the single rigid heddle offers.  Also, the same comment about the length of the warp applies.
You will also see something called a "backstrap" loom turn up on ebay now and then.  These are fun.  They use a rigid heddle and you tie one end to a tree, chair, porch railing etc and the other end to YOU.  You maintain the tension by leaning back.  Now THIS is the portable loom!
Another fun loom is the Inkle.  These come in various sizes, can be right-handed or left-handed or have both sides enclosed.  They are meant for weaving belts and sashes.  You can use your fingers to manipulate the warp for patterns or you can used weaving cards.  Card. or tablet -weaving involves threading your warp through a series of square cards with holes in each corner.  You create the pattern by turning the cards in a planned sequence.  You can also use a backstrap loom or get one of the dedicated card /tablet-weaving looms offered frequently on ebay.
The simplest looms you will see offered are peg looms.  These are simple frames with  pegs or nails at the top and bottom.  You manipulate the warp threads with your fingers.  These are the most basic form of TAPESTRY LOOM.
Then there is the ubiquitous POTHOLDER LOOMS!  They usually have loopers offered with them.  Great for the kids!
TAPESTRY LOOMS vary from very, very large looms with treadles and harnesses to a simple peg loom.   You will see the larger looms referred to sometimes as "Gobelin" style since they are based on the looms used in France.  Standing upright, they are referred to as "Haute Lisse" (or High Warp) whereas, if they are horizontal, "Basse Lisse".   Smaller tapestry looms, usually the rigid-heddle looms, often have available a stand for them.  Some have a way to attach your drawing or painting, called a cartoon,  behind the warp.  As to what to look for, any of the previous comments apply to the tapestry looms.  
If ANY of these looms are shipped knocked down, make sure you have or can get the assembly instructions.  A lot of manufacturers have their manuals online.
And, lest we forget - there are a couple of fun little gadgets that are often offered: the "WEAVE-IT" and "PURSE LOOMS."  THe Weave-its are various shapes and sized of small peg-type looms and the web is full of instructions for getting really nice textures and weaves out of them.   Purse looms are usually a block of wood with nails around the edges of the top.  You warp them by winding the warp thread around both front, back and sides, then weave around  top to bottom.  A wonderful little seamless bag is the result and the challenge is to create interesting colours and textures.
An odd "duck" that turns  up now-and-then is a "Hand-Skil " Nadeau loom.  This is entirely made of aluminum and has a drum attached to a steering wheel for raising the harnesses.  It was originally developed for physical therapy.  They are usually pretty inexpensive.  They can be quite fun to work with.

A few things to watch out for:  Again, make sure the frame of any loom you are buying is not warped or bent.  Beware of lots of rust.   Make sure, if it's shipped, it is properly packed and insured.  There is also a funny little gadget that turns up sometimes that appears to be a working miniature rug loom.  They will have a "half-finished" rug on them with a slogan like "Welcome" or the name of a family or company etc.  These are often described as working looms, in total innocence, for the most part, but they are NOT.  They are fun to own as decorative objects, in fact, I own a couple, but attempting to make them weaving tools is pretty hopeless.

I've been weaving half my life, which means a LOT of buying equipment over the years.  I studied at the Fashion Institute of Technology, worked in the textile industry, woven tapestry on commission, done free-lance sample-weaving,  and worked in textile conservation.  Most recently, I have been creating  and teaching in a summer weaving programme and  in my studio,  as well as turning out items to sell through a regional gallery, which means buying even MORE equipment.  So I've just about seen it all!  
Welcome to the world of weaving.  Have fun!