I don't think a fee-based upvote system can protect users from scammers who have enough resources at their disposal to artificially inflate their ratings [unfortunately, I can't suggest a better way to deal with such cases]... Would you mind sharing the terms of the smart contracts in an in-depth manner and will everything be fully open-sourced?
Only buyers would be able to leave a feedback on a product / seller, and a seller wouldn't be able to buy its own product. This means a scammer would have to create multiple accounts ( key pairs ), fund each address, place fake orders ( incurring fees ), wait through escrow periods (potentially mandatory for new sellers), and then give each fake orders a positive feedbacks ( fees again ).
I think that's a lot of effort, time and fees, just to scam one or two people and see negative feedbacks appear right after. Someone with enough funds wouldn't even bother doing so. Someone with low funds couldn't.
I think that the terms of the contract still need to be thought through and clearly articulated. The marketplace would obviously be open source.