There is a vast array of tanks on ebay, most of which are very similar, however there are a number of key differences that you should look out for:

1. Make, or manufacturer, usually 'Hen Long' these tanks are imported from China and, although they often work fine (as mine has so far), it should be noted that they are bottom of the range (or imitation) compared to makes such as Tamiya and Tokyo Marui (although these a rarer and often reach much higher prices)- see http://miniature-art.tripod.com/otherairsofttankreviews/id3.html for a comparison. In my experience, the Hen-Long finish is not particularly refined- or camouflaged with the Tiger
   
2. Scale- most tanks are either 1:24 - about 16" (40cm) long (13" not counting gun); 1:16 about 21" (50cm) long (18" not counting gun) or the huge 1:12 tanks at around about 30" or 76m in total (about 26" without the gun I think) there are also occassional 1:6 tanks but these are in a class of their own
3. Model- although, as far as I can tell, the actual model doesn't change the actual tank working (e.g. other than paint and body, tiger is fairly similar to panther), the are a number of versions out their, some with "deluxe" or "super" features including Metal tracks and metal gears which will undoubtedly make the tank last longer
4. Other Features- although virtually all tanks these days come with the standard 25 metre firing BB airsoft cannon (a very nice feature which, unless you shoot someone in the eye, is painless), they are increasingly boasting some more advanced features: Smoking Exhausts and "Realistic Sounds", both very nice features, but consider whether or not you need them as they will increase the selling price and also the chance of something going wrong if you ask me.
5. Postage- these are big things, so where possible I would advise you opt arrange for local pickup (most good sellers will oblige) and watch for those that say "see description as it will usually be in the region of £30
6. Sellers- not only are many of these tanks Chinese imports, but it is often foreign (namely German's) selling them so remember, if it doesn't work, returns could be a problem- see these guides for more advice RC Sellers, Tank seller.

So basically look around, and be wary but basically these beasts really are awesome when they work and I hope my experiences, and guide, have been of some use.