From the point of view of a buyer it's a great marketplace, with almost unbeatable number of good bargains at your disposal. Sure there are some limitations: You can't see the stuff you're buying (happens to get a sub-tascable book when you expect a large encyclopedia-like monster), but these are common for every on-line selling method.
Of course, as always, avoid beginners and inexperienced users, especially as sellers. They often are bunglers and very often dishonest.
The most annoying thing in eBay is that many power sellers (which seem to be "Powerful" from every point of view) don't accept discussion, are always in truth, and have a lot of means to prevent you from releasing a feedback and even can cancel your possible negative/neutral feedback(s).