There are several initiatives seeking to link 3D print owners with 3D print buyers, but as time passes only a few are going to succeed. One of them is likely to be 3D Hubs, currently the leader in number of participants.
The idea is pretty straightforward: if you have an idle 3D printer, sign up for the service. Then, if someone in your area requires a print they'll be able to find you via 3D Hubs. They'll receive a price quote and you can get some revenue for your otherwise idle machine. They call it "Local 3D Printing".
Currently 3D Hubs has around 1800 machines signed up for their service in numerous cities around the world. However, in order to encourage participation, the company doesn't "activate" a city until it has at least 20 participants. In this way they hope to have local 3D printer operators rustle up their friends.
They plan to include a review system soon, similar to eBay, where buyers can "rate" print operators, as well as a way for individual printer operators to have personalized "sites" within 3D Hubs.
They're quite big on the idea of community. They even have a concept of a "Mayor" for each city. The Mayor coordinates activities and meetups for the city and represents them with 3D Hubs.
The company is expanding rapidly and may even increase the rate as they've just opened a New York City office to build interest in the USA, where there are metric tonnes of personal 3D printers.
Via 3D Hubs