what would prevent a vendor opening up shop, listing a product or service very cheap; buying them on smurf accounts and leaving good feedback, and than raising the price to market value?
Edit: and wouldn't just adding an "amount spent" to the feedback page remedy this?
... Nothing. This happens and it is why you need to research your vendors beforehand.
OPSEC would be VERY important to keep their buying strictly separate from their selling.
This would require a bit of effort and quite a bit of time, and I think that's the only thing stopping them from doing it. In addition to if anyone found out they were fake they would be instantly marked as a scammer.