Ebay is where I get my out of circulation library books for really cheap. I just picked up a never loaned "Toward Artificial Sapience : Principles and Methods for Wise Systems" for $3.73. On Thriftbooks it's $156. Looking back, the seller was "BetterWorldBooks"!

I have ordered from Thriftbooks dozens of times. The only two things that happened was one package theft (obviously not their fault, but they re-sent the books for free, no questions asked), and one extra shipment of books I had not ordered (this was probably someone else's order that they sent me).