The issue in the US with accepting payments on behalf of sellers is that you have to then collect sales tax which is different for every locality. Then you also have to issue tax forms to the sellers for sales over a certain amount. Which leaves us with sellers collecting payments on their own which means using something like PayPal or Bitcoin. You would just have to have people build trust through reviews and hope for the best. You could base the service in a random other country so that you wouldn't have a bunch of regulations to deal with and leave all the tax stuff up to the buyers and sellers. You'd probably run into issues with accepting credit cards though.