So you have decided to cough up enough money to buy a small car, but instead you have to spend it on a graphing calculator. :-( Well, you should at least know what to look for when making your purchase, so you don't  get ripped off.
1) Avoid TI-92 auctions that don't mention that the calculator is a plus version.  The plus version added many features that demanding classes today take advantage of. The normal version isn't common anymore, so anything that you want from a friends TI-92 plus won't work right on the basic TI-92 most of the time.
2)If the auction includes an instruction manual, it is worth more because the logic behind these calculators can be extremely frustrating at times.  Trust me, I own one.
3)Obviously, any accessories add value, but be wary of some software, modules etc. because they could actually be freeware.  Cables are definitely useful, you can use them to get games and programs both from friends and from the internet.
4)If you don't absolutely need a TI-92, dont get one.  TI-86 is cheaper, and works well for Calculus and below.