Marco N.

There are weak forms of protection against scammers: the sole possibility is represented by Paypal, that is by eBay! The competition! It would be like buying a car from Renault and contacting Volkswagen for warranty service!
Personally I made several purchases (maybe twenty) almost all unsatisfactory, and I received only 5 positive feedbacks (many sellers forget to leave it) and two negatives - one because I dared to complain about a decidedly overgraded VG + disk - the retaliation system is routinedly applied, that is, if you leave a negative feedback for a dishonest seller, you get it back: such things happened on eBay many years ago.
A second negative was for non-payment: I had ordered (I think) 10 LPs but only 7 or 8 were available, the three that I was most interested in were missing, so I asked for the cancellation of the order but as a response I got a negative feedback. These things do not happen on eBay (I buy more than 500 euros a month on records).

PRO: Low prices, very good bargains, you can find titles/editions that are almost impossible to find elsewhere.
CON: No rating system for the sellers (no feedback can be released).

Most (98%) of books I've bought in the last 10 years (maybe 1000 in all) have safely arrived@ my home. Otherwise, I've obtained a refund either from the seller or from PayPal (but -again- this happens very seldom).
NOTE: Most of one-star reviews that I see here have been released by totally inexperienced users that do not clearly understand how abebooks works (e.g. - they expect to receive the book within 1 or 2 days when they've been sent as "fold of books" or by some other cheap method).

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).