Intro:
There are thousands of items related to paintball on ebay. From high end professional guns to lower end beginner entry level guns, paintball has a place for everybody. This guide will help you decide which gun you should go for depending on your situation. There are five major pieces of equipment required to play paintball; the mask, air source, paintballs, hopper (holds your paintballs), marker.
Although the level of seriousness for this sport takes into account what equipment you should buy, the amount of money you have to spend will also make a huge difference in what equipment you buy.

Mask (face protection)
The mask is the most important piece of paintball equipment. An appropiate mask for paintball will cover the entire face including the cheeks, jaw, chin, and most importantly the eyes.
There are multiple kinds of masks you can buy depending on your budget. Start by determining how much money you want to spend on your mask. Most beginners think that a mask is something they can go frugal on so they buy cheaper masks that fog up. Keep in mind, "If you can't see, then how are you going to hit your target?" A higher end mask with anti-fog lenses is recommended. Expect to spend about $30-80 on a mask depending on what you want to spend.
                Recommendations:
                   -V-Force Profiler ($80)
                   -Dye Invision ($50)
                   -JT Proflex ($50)
                   -V-Force Pro Vantage ($30)

Air Source
You have basically two choices: Nitro, or CO2. There are distinct advantages for both and you must decide which air source will be most favorable to you.
CO2 is the most commonly used propellant. CO2 has some distinct positive qualities. CO2 tanks and refills are relatively cheap, gets the job done, and is readily available in almost all paintball pro shops. Most guns such as a Kingman spyder will handle CO2 with no problems. CO2 is unfortunately a fairly dirty air source because rust, debris and other junk gets into the tank via fill stations. This could cause a shorter life span of your gun but often it is an insignificant problem for most people. CO2 is a fairly unreliable air source because paintballs will travel at varying speeds and distances. This problem is significant enough for some people to get a more reliable air source: Nitro.
Nitro is basically the air we breathe compressed into a tank. It is far more reliable in a sense that each paintball will travel about the same distance per shot. Although nitro is an ideal air source, there are some negative qualities. Nitro tanks are very expensive. For comparison purposes, a 20oz. CO2 tank will allow about 1000 shots and it costs about $30. A 68/4500psi Nitro tank will allow about 1020 shots and costs roughly $150. Not every store has air compressors used to fill nitro so if you do not have a local store or field that allows for nitro, then  go with CO2. If you can get nitro though, then by all means think seriously about investing in a nitro tank. When you are finished with paintball, you can resell that nitro tank on ebay. CO2 tanks will probably have to be sold with your gun since it is rarely worth it to sell it individually.

Paintballs
Paintballs come in many colors, sizes, prices, and quality levels. The type of paintball you use will heavily determine whether you spend more time playing and shooting or cleaning your gun.
Many people think that cheap paint is "good enough". Cheap paint is often filled with a lighter watered down fill and the shell is of very poor quality. Cheaper paint often breaks inside the barrel or breech of the gun and causes a messy and frustrating paintball experience. If a paintball explodes inside the gun, it must be cleaned otherwise performance will be heavily compromised. Cheap paintballs are also usually abnormally shaped and even the slightest imperfections can cause breaks insid e the barrel. Most of the time, cheap paintballs will break more often in your gun and barrel than on your target. cheap Brass Eagle paintballs is an example of very poor and cheap paint.
Higher end or brand name paintballs are recommended. RPS/PMI, Diablo, and Evil are companies that produce higher quality paintaballs. High quality paintballs often do not break inside the barrel and gun but break often on the target. Higher quality paintballs will also generally be very round and close to perfection. This allows for straight shooting and more consistent distance and speeds of the paintballs.
                Recommendations:
                   -RPS Marballizer ($60 a case)
                   -PMI Premium ($50 a case)
                   -RPS Rampage ($40 a case)
                   -RPS Big Ball ($35-40 a case)

Hopper
A hopper holds your paintballs and once again depending on your budget you must decide which one to get. There are higher end electronic hoppers all the way to the standard 200 round gravity feed. If you have an electronic gun, an electronic hopper is heavily recommended. If you have something like a tippmann 98 custom, a 200 round gravity feed hopper should be sufficent. Keep in mind that if your hopper does not feed paintballs fast enough into your gun, then your bolt might chop paintballs right in half as they enter the breech.
Agitated and force feed hoppers are a must for fast guns if you want to shoot fast.
                    Recommendations:
                     -Halo B ($100)
                     -Reloader B ($100)
                     -Viewloader Evlution 2 ($60)
                     -Viewloader Revolution ($20-30)

Marker / Gun
Depending on your purposes and budget, the paintball marker should be chosen carefully. There are many guns out there to choose from. For higher end guns ($500+) then it's more about which one you like the feel and looks of. Those guns usually require compressed air (Nitro).
For the lower and mid end guns though, there are once again many choices. Here is a breakdown of good guns by price range.
                $500+
                   -Smart Parts Vision Shocker
                   -Angel Speed
                   -Angel G7
                   -Intimidator
                   -EGO
                   -Proto Matrix
                   -Dye Matrix
                   -High-End Autocockers
                $300-499
                   -Smart Parts Nerve
                   -Smart Parts Impulse
                   -Worrgames Superstock Autococker 05
                $200-299
                   -Smart Parts ION
                   -Diablo Wrath
                   -Tippmann A5
                   -Worrgames Autococker
                   -Evil Omen 2.0
                $199 and below
                   -Indian Creek BKO
                   -Tippmann 98 custom
                   -Electronic Spyders
                   -Mechanical Spyders
                   -Diablo Mongoose LCD

Paintball is extremely fun, but is arguably one of the most expensive sports to get into and play. Make sure you play safe, never take your mask off while on the field (accidents happen!) and keep your barrel sleeve on at all times until it is time to play. There are some other pieces of eqiupment not covered in this guide, but it is meant more for a basic beginner's guide. The forums at pbnation.com is heavily recommended if you run into any problems with your equipment or have any questions about paintball in general. Good luck and most importantly play safe.