When buying a paintball marker on ebay, one should consider the following:

-Length of use
-Level of play (hobby, amateur, semi-pro, pro)
-What accessories, if any, come with it
-Protective clothing needed, if any
-Price vs. Deal

When considering the length of use, you will want to see how many games or how long you will play.  If you are going to play three or more hours, you need to get a marker which will stand up to extended bouts of rapid trigger work and firing.  One which won't also break alot of rounds in the barrel.  This will make the difference between you or your opponent leaving the field of play.  I am using a $30 Wal-Mart special.  The person I was facing off with had a $150 Tippman with a long, quiet barrel, and you could hear my bolt fire off every time, so you knew what was coming.  Needless to say, to fire was to give away my location.

For the hobbiest like myself, I suggest finding a marker with a full field kit in the $75-150 range.  This kit should include the marker and barrel plug; at least a 100-round hopper, usually filled; an empty 9 or 12 oz CO2 canister or cartridge system; a face mask; and in some kits there might be gloves.  Now, the more expensive the kit, the more or better options will come with it, how to decide which is right for you depends on other factors, once again the length of play, and level will dictate which you will need.  Lets say theres a kit with a marker, plug, 200-round hopper, 12oz co2 canister, mask, and gloves, its $150, and its all steel construction.  And theres another that comes with a plastic marker with a steel barrel, 100-round hopper, and a cartridge system with two cartridges, and a mask for $50-75.  Odds are the more expensive is the better bet, because the all metal construction is more reliable and accurate.  So once again it depends on your level of play.

Depending on your upper levels of play, if you are in a competition circuit, this isn't cheap.  The cheapest gun I found that is really good for tournament play is $250 with no accessories.  Now this is something I have no clue on, but some of the same ideas above apply.

Now if you are the guy who gets the full field kit, you will not need much more protective gear.  But the idea in the field is to blend in, well, for those who don't play speedball.  So if you have a black mask, black gloves, or camoflauge versions of either, all you will need is a set of BDUs (Battle Dress Uniforms) that you can get from any army surplus store or hunting supply store.  These are better to buy online, or in surplus because they run cheaper.  Expect to pay $50 for a jacket top, pants and a shirt to go under, my advice is to skip the shirt, because as long as its closed, who cares?  Most sets are adjustable, but you still need to pay attention to size as the pants and shirts only fit certain chest and waist sizes.

Now in price vs. deal, you want to find the most for your money, now if you find a plastic marker, with a 20 oz CO2 canister, mask, gloves, plug and 200-round hopper for $75; or if you find an all-steel marker with plug, 12 oz canister, mask and 100-round hopper for $115, the steel one would be the better deal.  Because of the rugged construction it will last longer and play harder, and most come with a two-finger trigger.

Accessories you might want to consider are a long barrel with a silencer on the end, an auto-load hopper which uses an electronic eye to sense the presence of a ball, and if it doesn't, it agitates the contents of the hopper until it shakes them in the tube; also, get a belt pack that can hold up to four tubes of rounds.  Usually these only hold 100, but this can make the difference on the field, because if you run out, and you are hunkered down, you have no choice but to go out.  On some markers, you can get a remote kit, which puts a pack on your back with your CO2 bottle in it, and a hose connects to your marker.  This is expensive and considered only for those who jaunt between hobbiest and competitor.  Of course you can get scopes, lazers and such, but why? That takes all the fun out of trying to hit the competition.  Also consider anti-fog cleaner for your lenses.  This helps loads!  A person I used to play with had a small CPU fan on top of the goggles and cut the tiny holes out of the goggles, mounted a 9V battery and a switch on the side of the mask where it wouldn't interfere, and it helps to blow outside air in the mask to defog it, or it can be flipped to exhaust heat from the goggles.  Now those who know their way around electrical circuits will want to try this.

I hope this helps you in choosing your next paintball marker!