I used to sell books a little. I'd go to estate sales and the like and buy books cheap and put them on ebay or amazon and wait. If you know what you're doing, and have a little luck, it can be fairly lucrative I had several books that I paid under $3 for that sold for over $50 and a couple that went over $100. I also had a lot of books that I paid a dollar for that will eventually be donated to goodwill. Knowledge, of course, is the key, and kmowledge is often hard won.
Sometimes luck will do though. When he was 16 Capt. Nathan Harlan of the Indiana National Guard, picked up an old leather bound copy of the Federalist Papers for $7. He bought it, in part, because he was studying the Federalist Papers in school. It tTurned out to be Volume I of the two volume first edition. Facing his second tour in Iraq, he decided to cash it in, thinking he might get $500 for it. He got $80,000. As nice final touch, the auction house, Heritage Auction Galleries is waving its 20% fee in recognition of Capt Harlan's service in Iraq.
Hat tip to Ace of Spades HQ.