To put it plainly, no reputation system is resistant to a Vendor purchasing their own items and making false-positive ratings.This is especially true of a pseudonymous decentralized marketplace, where Buyer identities are — by default — undisclosed. Even a web-of-trust model, which is excellent at detecting suspicious islands of ‘reputable’ users, will not be able to distinguish between real and fake ratings of a Vendor.
Maybe all reputation schemes that try to assign a globally valid rating to users will be bad. Perhaps the only solution is to let the context fall and do not perform any kind of calculation/assignment.