or you could invest in PART, which is doing a real decentralized marketplace, already live  (but still beta-ish)

New version "infinite markets" should be out in march/april, and will enable the ability for anyone to create any number of private or public marketplace.

You can use it on Tor, every image is stripped of metadata, payment is done anonymously (ringct) and orders are only done between seller and buyer (nobody else know)