Why isn't a basic version of proof of work a reasonable approximation of that for the time being? Ex. to provide a review you must also provide a value that when postpended to your review gives a hash containing some properties that scales with the number of reviews. This doesn't solve decentralized reputation at all but does prevent review bombing.
If the buyer leaves a review after making a purchase, that's effectively a small proof of work in and of itself, and you can only leave one review per purchase.
We haven't seen review bombing attempted yet, but if someone were willing to pay a bunch of fees then I suppose it is possible to do.
In book publishing an author can purchase copies of their own books in order to increase purchases by others. This effect is actually common in many industries, even in mobile apps :)
It's also borderline an unsolvable problem, because how do you identify "legitimate" customers in an anonymous de-centralized system, when it's even too complicated to achieve in a centralized system?
There is a dichotomy in real cost/benefit ratio for PoW systems as spam control between real users and attackers. Attackers have more to gain from accepting the PoW penalty than users do. There is no difficulty of PoW that would both deter attackers with a financial motive and be reasonable for average users.
