There are some things that one just assumes everyone knows. I've worked hard at learning not to do this over the years, but I still have a long ways to go (and I don't really ever expect to get all the way there). Chatting with a newly-made friend today, I found out that she didn't know about all sorts of online resources to buy books with that I sort of assumed everyone knew about. So here are a few links I know of that I find helpful when shopping for books. If anyone knows any I left out, please leave them in the comments!
The first and most basic tool I use are two sites which themselves aggregate other online search engines. There are more than these two sites, but in my experience checking too many one hits diminishing returns, since they all check more or less the same databases. But there isn't complete overlap -- sometimes one will have a surprising hole (e.g. not finding all editions of a book but just some, not finding a book on some site that the other metasearch engine will find it on, etc), so it's worth checking more than one. So I use these two:
These sites aren't perfect -- the errors mentioned above will sometimes occur on both -- but in general they do a fairly good job, I think.
Most of the rest of the sites I know of are ones which those two will point you towards. But still, for anyone who wants to check them directly, there are a number of sites which allow you to search many used book stores across the U.S. (and, often, in England & elsewhere -- pay attention to where it's coming from, as it may affect shipping time and/or price!). The biggest of these, I believe is:
--but another one is:
And Amazon.com, of course, does the same thing, as well as selling you new books (and barbecue grills and underwear and probably short-range missiles). Finally, while not strictly speaking just for used books, a lot of used books appear on:
The other thing to take into account, of course, is shipping cost. Most of these sites will give you a discount if you buy more than one book from the same store (i.e. the same actual, physical used book store, not just the same online portal) so it's worth searching around to see if you can combine books. In general, abebooks has somewhat higher shipping prices than alibris, which in turn is a bit higher than half.com; and if you find a book on one, it's worth checking the others since the same copy is often listed on multiple sites. On the other hand, I often find that ordering books from multiple stores is cheaper than combining orders, even factoring in multiple shipping costs.
For new books, overstock.com has Amazon-like prices on a lot of things, but only charges $1.40 per book for shipping, so if you're buying under $25 (the amount at which amazon will ship for free), it's worth trying that. In contrast, ebay often has outrageous shipping prices -- a lot of ebay sellers seem to fold a lot of their actual merchandise price into their shipping cost (presumably to hide it when sorting ebay by price or on search engines such as the above, or simply to try and sucker people who don't pay attention), so pay particular attention when buying books on ebay (this is even aside from the auction factor -- I'm talking now just about the ebay "stores" which sell you things immediately.)
Those are some of the web sites I use. Again, if anyone has any others, please leave them in comments!