How about this:

1. Have a way for buyers to report when a seller is bribing them to fake reviews. If the report turns out to be genuine, give the buyer a small reward ($5/10).

2. Have Amazon occasionally pretend to bribe people to do fake reviews. If a user does the fake review, their account is banned for one month. If the user reports the fake review, they get the small reward.

Between the carrot of reporting fake reviews, and the stick of being banned for falling for the request, you'd "crowdsource" a lot of the issues.