Personally, I like going to Half Price Books. They have an incredible selection and a lot of (DFW) locations. Unfortunately, I think they are a local chain here in Texas only. There's another amazing used bookstore in Denton, TX called Recycled Books. I spent many hours there in my youth!
Of course, eBay and Amazon always have "pre-loved" books worth buying if there are no good local stores.
So awesome to have local book stores to go to! :D