I ran idea 2 with my in-house shopping expert (gf) & her first reaction was 'This is so old-fashioned, plenty of people are already doing that on bbs/forums'. So people are already doing it, how are you going to make it better? http://www.dolc.de. (in the second-hand forum / Chinese)

What about taxes? Fraud? You'll need a solid reputation system. I like the idea though.