Tindie is a marketplace: many-to-many. 4pcb.com is not a marketplace it's a manufacturing service. Shapeways is a both a marketplace and a manufacturing service. The beautiful thing about their marketplace is that they also provide the manufacturing (3D printing). So as a seller all I have to do is provide the 3D model STL file and people who buy the product get a physical object. It would be awesome if Tindie did the same thing with PCBs. Currently I don't know of any PCB marketplaces that provide manufacturing as part of their offering. Sparkfun will sell products for you but there are minimum orders, and I believe you have to purchase all the raw materials ahead of time. It would be awesome to have an appstore for PCBs. If you make one, email me and I will be an evangelist customer.