Laen has just opened up an online shop for PCBs created through the OSH Park service. The PCBs are sold with permission from the creators. While some of the PCBs may be on hand, it looks like many will be added to the next PCB order once you order it in the store. This is a great idea, and I hope everyone who submits their board to OSH Park opts-in.
UPDATE: Looks like the store is just a place for Laen to unload extra PCBs. For a maker marketplace, please checkout tindie.com.