Since AuctionNinja is a bidding platform licensed by individual vendors, they are required to maintain their own set of client reseller certificate records (for IRS and state tax purposes). If they get audited, they need to produce their own documentation. As a result if you are a bidder and have a tax exempt / reseller's certificate, you need to submit that to the individual vendor/licensee. That means, if you bid in three auctions offered by three different vendors, you will need to submit your resellers certificate to each vendor.