So mark a vendor as "new vendor", add mandatory ghost shopping to all product skews (postage returns paid by vendor), funds only clear after product arrives + n

So mark a vendor as "new vendor", add mandatory ghost shopping to all product skews (postage returns paid by vendor), funds only clear after product arrives + n days for new accouts, etc to build in security that products sold are legitimate. All you need to do is make cost of standing up a scam more expensive than the real thing.