>Say you want to be able to sell prints of your great photos //

This opens the printer up to potentially massive copyright infringement issues. Sharing images online is one thing, being directly paid to actualise then on offline products is a big step with a lot of potential liability.

There's already lots of competition in that space, the sites I've used for Tshirt & mug printing (in Europe) have been very hot on trademark and copyright issues, stiflingly so IMO.

There's probably much more to be made selling pictured goods over good with pictures on.