I think a web-of-trust-style reputation system would work well here, since "votes" cost money. The reputation of a buyer is the mean of the reputations of the

I think a web-of-trust-style reputation system would work well here, since "votes" cost money. The reputation of a buyer is the mean of the reputations of the projects they've backed (times some confidence metric). The reputation of a project six months after it started is 1 if they kept their promises (as measured by confirmation emails sent to the backers) and -1 otherwise. The reputation of a project before six months is the mean of (the mean of the reputations of its backers (again times some confidence)) and the reputation of the person who started the project.