Clever used book arbitrage:

I make a living buying and selling used books. I browse the racks of thrift stores and library book sales using an electronic bar-code scanner. I push the button, a red laser hops about, and an LCD screen lights up with the resale values.

It feels like being God in his own tiny recreational casino; my judgments are sure and simple, and I always win because I have foreknowledge of all bad bets. The software I use tells me the going price, on Amazon Marketplace, of the title I just scanned, along with the all-important sales rank, so I know the book's prospects immediately. I turn a profit every time.

(Via Marginal Revolution.)