The best thing I ever found was a complete in box copy of Dragon Warrior IV at a goodwill. Everything inside was in perfect condition except a small tear on the corner of the box.
Then I got the Descendants of Erdrick to sign it! Another great buy was a complete in box copy of Final Fantasy II (IV) that had EVERYTHING INSIDE! Spent $26 and was soooo psyched about it. I ended up giving it to a friend of mine who valued it more than me. (I was young, didn't think about it, and super generous obviously.) I know he still has it and offered it back if I ever really want it.