This is kinda a niche thing, but there is a large assortment of smoking pipes for sale here, and so far every pipe I've bought, I got here.  I'll keep this simple as possible. Before I get started though, this is mainly geared towards pipes for smoking tobacco, and not certain other herbal substances.  While it probably is possible to use something apart from tobacco, there are specialty pipes for that, which I don't think I have to describe (chances are, if that's what you're looking for, you've seen/used one already). 

When buying a pipe check for the following:

-What size is the bowl?
Really, you're probably looking for something between an inch and 2 inches deep.  Pretty hard to find one outside of this boundary, seems like.  Diameter wise, I'd say most tobacco pipes are about an inch and a half, give or take a centimeter.

-How long is the stem? 
In general, about 5 and 1/2 inches is an average length but some are shorter or longer.

-What is the pipe made of? 
There are 4 main ones you'll find here: 

• Briar- the best in my opinion, it's durable and builds cake easily
• Cob/Corncob- cheaper, and good for testing a new tobacco, but also very fragile
• Meerschaum- kind of a middle ground between briar and a cob (I think, I've never actually had one)
• Metal- should be self-explanatory (this is what most smokers of something...other than tobacco are going to be looking for probably). 
• I think I've seen some clay pipes also but I wouldn't recommend one for actual smoking.

-Is the rim (top) of the bowl charred? 
Many careless smokers (myself included at times) will hold the lighter to the bowl for too long, which burns the briar (or whatever the pipe is made of).

-What condition is the bit in? 
The "bit" or mouthpiece of a used pipe is pretty much GOING to have bitemarks, but make sure the bit is not chipped (it'll make it easier to get that awful tasting juice from the tobacco in your mouth if the hole in the bit is too large).

-Does the stem fit inside the shank properly?
My most recent pipe purchase failed this test.  If the stem is removed from the shank (which is where the stem fits into the bowl) while the pipe is still warm from smoking, it can loosen, and it's truly a pain if this happens.  And also someone who isn't keen on pipes might mix stems and bowls, this is not a good idea as every pipe's stem is made for that bowl.  Bending that rule can mean a loose stem (the bowl easily falls off of the stem on mine, actually).

There are other things to look for if you're a collector, but chances are you know what those are and I won't go into that, as this is intended for someone who simply wants a pipe to smoke.

A word of warning, it's probably a good idea to soak a new pipe in some kind of alcohol (preferably vodka since it's safe to ingest) just to be sure.  I'd also run two or three pipecleaners through.  If you buy a pipe that's been sandblasted, this is a must unless you like the idea of getting that residue in your mouth when you light up. 

One last thing, and some pipe smokers would probably argue this point with me, but if you buy a used pipe and it already has a cake in the bowl, scrape it out.  You'll want to start fresh with your own tobacco blend, and build the cake back up with your smoke of choice.  It's a minor difference in taste but it is there, but on the other hand it's not a necessity.  (BTW "cake" is where ash builds up into a hard layer around the sides of a bowl, it helps keep your pipe from burning too hot and prevents cracks)

A search for pipes here on eBay to get you started