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.
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.