If you're attending the University of Calgary, don't get your proverbial knickers in a knot over buying textbooks. With the right know-how and a little sleuthing, not only will your book buying be trouble-free, you can also save a few bucks along the way.
You're really stuck with two options: the university-run new bookstore, or the Students' Union's used bookstore. Each have their advantages and disadvantages for both buying and selling books.
The used bookstore is a crapshoot because there is no real guarantee anything you're looking for will be there, nor will the current editions always be in stock. But if you don't mind sifting through piles of books, and aren't bothered by occasional highlighting or notes in the margin, the SU's bookstore is a good starting point. Prices are consistently cheaper than the university's store, and if you get there early (if you're reading this and you haven't checked, you'd better hurry), you can usually find a large number of your books. Just be sure you're buying the right copies and editions, as there are no returns.
The new bookstore has the obvious disadvantage of paying full price for textbooks, but you're almost always guaranteed your books will be there. They even have the occasional used book, which generally cost more than the SU's store, but offer on-line shopping, a frequent buyer program for non-required books, and worry-free returns (up to 14 days after the start of classes) if you've purchased the wrong book
Selling books is also a complicated game. At the used bookstore, you set the price and keep 80 per cent if it sells, but this can sometimes take months. On the other hand, at the new bookstore you get cash right away, but you'll receive much less (50 per cent at most). In many cases, they won't take your books at all if it's out of date, damaged or if they're overstocked.