Exactly once in my life I bought something which turned out to be an awesome investment. I purchased a discounted book back in 1998 or so, and now it's become rare. I think I paid about 6 bucks for it originally. I've got it listed in eBay now and it's up over $20 with still a day and a half left to go. I didn't like the book all that much (it's a fantasy novel), so it doesn't bother me at all to part with it.