Textbooks are expensive. So, whether you're selling or buying you have a number of options:
1) University Bookstore. As stupid as this sounds, if you have a "custom" edition for a book, chances are, you're not going to find it online for some ridiculously cheap price. Also, they have the buyback option, which works well if no one wants to buy your book back online, and they give you the 10% patronage refund at the end of the year.
2) Contrary to popular belief, some of the *big* websites have a wide variety of good textbooks. since they're widely heard of and respectable, amazon.com and http://www.ebay.com have some good deals. I once bought a textbook for <$20, shipping included, from ebay. Don't underestimate them.
3) Some of the others have posted websites that specifically sell textbooks, http://www.half.com or http://www.powells.com can also have some good deals. Also, http://www.ecampus.com
4) Someone commented earlier about http://www.thehuskieweb.com, which I hadn't heard of up until this point. It seems like a good place to start. Especially, if you're trying to avoid shipping fees.
5) One other option to avoid shipping costs is http://www.shukoo.com, which I've *just today* bought two books from. I'll report back on how successful that was later. But shukoo also shows search results for a variety of websites.
6) And if you don't want to stray far at all, you can always post something to uwtextbooks, and advertise to other lj users. Seeing as you're here anyway, it won't hurt to check that out.
7) A rather dead place to sell your textbook, if you're planning on leaving it there for a while, is https://students.washington.edu/uwisa/textbookexchange/access/ . I think I might have gotten this site from an email or something. But, no one has bought a single textbook I've put up there, and no one has responded to any of the textbooks I want to buy. I'm really not going to recommend it, but I kind of feel like it deserves some sort of mention.
8) One more note: if you google "textbooks" you come up with pretty hefty list of places that also sell textbooks, like http://www.directtextbook.com/ or http://www.bigwords.com/
9) My roommate's been telling me about how some people post their textbook listings on a bulletin board in the HUB. I can't confirm the current existence of said board, but it's definitely been there before, because I know a lot of people did it.
If you're buying a textbook, any number of these options are probably good, especially if it's shukoo, or thehuskieweb where you can probably just meet them at the HUB to exchange money/books and therefore not pay for shipping.
If you're selling a book, then it's probably easier to just pick a dozen places and advertise the hell out of your book until someone decides to buy it. You won't really have to worry about shipping since amazon and ebay make the buyer pay for that.
I hope this helps. And comment if you have any other suggestions, or "reviews" for these sites. Perhaps the order in which I've listed them? I just kind of did this as I thought of it.
Think I should email the moderator(s) and ask that they put it on the userinfo instead of memories? *g*