This was posted a couple weeks ago from the founder of a failed parking startup in Boston: http://chrishoog.com/from-beginning-to-end-the-story-of-a-fa...

tl;dr - The demand for this far outstrips the supply of people willing to rent their spots to strangers for a few bucks.

HelloParking tried and folded, while ParkCirca is still going (http://www.parkcirca.com/) but has sparse listings. Tough space, apparently, or perhaps just still nascent due to lack of consumer mindshare.

It wouldn't scale without a significant amount of synergies.