Starlight - Thanks. But don't be put off by how much effort it might take, it's absolutely worth it - and it's really not that much effort. The book stores generally don't change their prices from semester to semester, so it's not like you need to check every time how much the book costs, just remember what you bought it for and sell it for a little less. Most of the effort comes from taping flyers to the doors, and well, if taking an hour to walk around campus can save/earn you two hundred bucks, seems like it's worth it.
I wish everybody resold their books like this because its really win-win. The seller wins because he gets way more than $20 for a book he originally shelled out $80 (or more) for, and the buyer wins because he's paying $50 or $60 instead of $80 for the same book. The only loser is the bookstore, who's ripping you a new one anyway.
The principle is simple: The bookstores might charge $70 for a used copy of an $80 book, and you can always charge less than the bookstores and get more than what they would give for a buy-back. Even for novels, let's say new costs $8, used $6, and buy-back price is between $1-$2. Why not sell your copy for $5 or $4? You still come out ahead of the alternative.
Sorry this post is getting so long, but if everybody re-sold unwanted books like this we'd all save a lot of money, and be able to re-sell the books we bought for almost the same price we bought them, and with all the saved money, maybe we could have all eaten something nicer than ramen noodles every night for four years.