Flip Craigslist on its head. Post things you want to buy, the service fills in questions and details on them using a combination of Mechanical Turk workers and social crowd sourcing. Looks like Reddit, with comments even, has Buy Local™ and category browsing options. Emails/SMSs you when it finds things that match your 'buys'.

Subset idea: comments on Craigslist postings. "Went and saw this couch. It's huge and hideous in person."