The one question I had to myself was: whats stopping these sellers from signing up as buyers and performing a 'fake buy' to leave themselves more positive feedback? I'm no expert on this, but wont Tor itself will prevent basic security measures other sites may use like logging account IP's etc...
They would pay for the review the SR fee, which is omething like 10% I guess. So yeah, you can buy yourself some positive feedback, but I'm pretty sure it doesn't make economic sense.