The bookstore is kind of hit and miss. Most of the time you don't get that much, but I had a couple books last year that I ended up getting more back for than I spent on them in the first place.
There are reasons the bookstore is hit and miss -
-If the book is not being used the next semester, the bookstore will not buy the book to be resold. If the bookstore doesn't buy it, the book wholesaler will buy it at lesser value. Most bookstore managers I know don't want to warehouse books over the summer, etc.
-If the professor switches books to a new edition, the bookstore will not buy the book since it will not be used on campus the following semester.
-If the bookstore is buying but has met their quota for the next semester, they will not be buying, but the wholesaler will, so you will get less for the book.
-Iowa State doesn't actually run their book buyback, they have a wholesale book company run it. The wholesaler is buying for ISU based on what the bookstore told them to buy first. If the ISU bookstore has no use for the book, the wholesaler then buys it.
-Bookstores typically pay 50% of new price when they buy the book, and wholesalers pay roughly 25%. Although this depends on the national demand of the book.
-While international versions of textbooks are cheap, they have no value to college bookstores, or book wholesalers. Your best bet is to sell them yourself.
Best tips when selling books to a college bookstore
-Find out if the book is being used next semester, if it is and you can sell it the first day, you have the best chance to get more money than waiting later in the week.
-Ask the bookstore how many copies of a book they are buying - my bookstore always posted how many of particular titles we needed, especially books that were hard to find.
-Getting upset with the buyer is not going to get you more money. The bookstore only orders the books they are instructed to order by the Professor. If you want to get upset with someone, talk to your professor - they make decisions on what textbooks to use. The bookstore personnel doens't choose books for professors.
When I was a bookstore manager, I hated book buyback because someone always got ticked off or felt like they were being ripped off or forced to sell their books. I was told them - nobody is forcing you to sell your books, and if you don't like the value offered, don't sell it.
As for online, I personally sold a ton of grad school books on Amazon. Very easy to use, they give you a credit for shipping (media mail rates) and it is easy to get your money.