Is there a reason you want/need to be the commerce platform on which these resale transactions take place? Two-sided markets are generally more difficult to cr

Is there a reason you want/need to be the commerce platform on which these resale transactions take place? Two-sided markets are generally more difficult to crack, and displacing eBay in particular is going to be challenging. I'd expect your cost of customer acquisition to grow quite significantly once you've tapped out the low hanging fruit. If your core competency is your ability to detect counterfeits, however, maybe there's an opportunity to convince legitimate vendors to pay you to certify that their items are authentic, something that they then promote to prospective customers in all of their sales channels. You'd probably need to offer a buyer money-back guarantee for this model to be viable, but if you did this, you could try tying your pricing to the vendor's sales price (i.e. it costs more to certify a handbag being resold for $750 than a dress being resold for $250). If you're wedded to the idea of running your own commerce platform, you could provide vendors that certify their goods with you a free listing on your site, and reimburse the certification fee from the sales commission if their product sells through you. By getting listings this way, you can avoid some of the questions you're bound to be asked if your only proposition is that you're an eBay competitor (think "How many sales do you generate a month?"). And you avoid having to grow your buyer base to make money.