http://whipper.abebooks.com/abep/il.dll
  http://www.clark.net/pub/rmharris/cgi-bin/wwwwais.abaa
He has bought dozens of books this way, from $10 paperbacks to $100's
rarities.  About two weeks ago I asked him if he could find an obscure
maths book for me.  I thought the chance of finding it at all were
pretty slim but he came back with a bunch of places to order it using
the above URLs.  New or used?  Fine or worn?  $20 or $50?
Definitely good news for book-lovers everywhere!
Rob.