A multisign service like this would be very difficult to make worthwhile. On small transaction, it would be hard to justify doing an investigation for a rate that would be worthwhile to the parties involved. It works well for Visa because Visa's already handling everything. Having these investigation services encourages people to use Visa, due to the peace of mind. It comes with it. With this type of service, you have to seek it out, pay for it separately and pay enough that this party will actually do an investigation. Obviously, it would have to work like insurance, were a majority of people aren't being scammed, but it's a difficult business model to comprehend.