There is a large element of "caveat emptor" involved - always check the postage / delivery costs, whether buying on ebay, Amazon or other sellers.
It doesn't matter whether you are buying a book or a plasma TV - very often the delivery charge can make the cheapest item a lot less competitive. So the total price is the only one to go on.
There is often the opportunity for feedback - not just on ebay. Amazon sellers (and ABE also, I think) have ratings and feedback too.
It's not always in the vendor's favour, though. I bought a virtually new computer book on ebay - RRP about £25 - for 99p, and the vendor charged £5 postage, but it actually cost over £6 for delivery (he sent it 1st class, not standard parcels). So by the time he had paid his ebay listing, selling and PayPal fees, he had made a 150% loss!
You can't have everything - where would you put it?