One person mentioned addall.com but what's really great about this site is that it COMBINES many of the search engines and searches them too- like abebooks, alibris, amazon and amazon.uk, amazon.ca, Powell's and Strand Books, half.com, Barnes & Noble, Books a Million, Hamiltonbook. com, biblion.com, overstock.com, ILAB-LILA- they're all searched in one search at addal.com- you have the best selection of used books and when you pick one you like for price and condition it takes you to that particular search service's site (abebooks, alibris, etc.) and you deal with them.
For you in the US (they don't ship out of the United States) you gotta check hamiltonbook.com. They also put out a monthly catalog which is the best I've ever seen or dealt with and I've buying books from them since 1984. This is a REMAINDER house- but much bigger than Barnes & Noble as far as the catalog- the books are all NEW (though some are a few years old- they never left the publisher till they were remaindered because their sales droppped off. They are up to 75 % off retail price. I've bought so many books from over the years- you usually can beat used bookstore prices with them- and no matter how many books you order they only charge $3.50 shipping total! (it used to be $3.00 for years). Try finding THAT anywhere else. Everything is remaindered sooner or later. If you get on their catalog list you won't be sorry. The end of year catalog is the best- most publishers are on the Jan-Dec fiscal year so they clear out all the slow sellers before they have to pay income tax on the inventory at year end.
Good book hunting, folks!
Where liberty dwells, there is my country.