Somebody found a photo of mine on flickr that they really liked. Complimented me on it extensively and said they’d like to print it for their living room. I remembered I have a paypal account set up (for the occasional selling of crap on ebay), named a price and had them transfer money to that account. They agreed to the license restriction, made the payment instantly, and I forwarded their print. Few hundred more of these and I’m in business!