There are also few sites that let you rent the different sets. My brother's used NetBricks and it sounded like it could be another option if the buying/selling thing makes it too much.

From what he told me is you pick a set/s online, the kit comes in the mail, build and enjoy it for a bit, then you would tear it down completely when you are bored of it, stuff all of it in a special zip lock bag and send it back for your next set. He told me they go over all the returned pieces to look for damaged/missing parts, "sanitize" (probably just wash them) them, and repackage them for the next renter.