Couldn't this be solved by introducing a financial (dis)incentive system?

I could imagine a system where you pay very little for casting a balanced review and you pay extra for casting a very positive or negative review. Cost per review would have to be adjusted according to product price. (perhaps a percantage of average market price?)

This introduces cost to everyone and increasing cost to both fake positive teviews and negative review bombing thus making it less likely. This cost for reviewers could be mitigated via zaps or zap-like alternatives for actually helpful reviews, which, if actually helpful could even be profitable for the reviewer.

If needed it could be coupled with (some parts of) reputation systems like the one on Stack Overflow.