That's why I do the library!! I can return them, or keep them longer if I need more time! Also, I miss the used book stores, where you can take in books you've finished, and get credit for them towards the next stack you buy. Do they have stores like that here?
My library doesn't have a huge selection, unfortunately. That's what you get from living out in the middle of nowhere. They can order books for you from other branches, but they charge for that service.
Most of the books I've been buying have been from second-hand shops or carboot sales (kind of like big flea markets). I do get tempted when I'm in a new bookshop, but I try and resist! At a carboot sale, most of the books seem to be 3 for £1, which isn't too bad! I'm not sure about selling books back or getting credit at second-hand bookshops, but there must be places like that here.
I'm kind of a pack-rat when it comes to books, so if they're good keep-able books, I tend to want to hold onto them. If they're junky paperbacks that don't mean that much to me, I sell them on Amazon when I'm done with them. So far I've done pretty well!