One (partial) solution would be to simply eliminate reviews. They're compromised. They aren't worth anything anymore and are just taking up bandwidth and bothering users. Not to mention fostering a weird little black market.

Just make people research what they're buying elsewhere--which will probably also get swamped with fraud--or just let customers roll the dice without the false sense of security. They're already doing it, but with a bunch of fake 5-stars instead of a bunch of unreviewed items.