The big question that is always a challenge for a marketplace is refereeing between buyers and sellers when there is a conflict. Basically, what processes are in place to make sure there is a resolution if a transaction turns sour between buyer and seller. Ebay, for example, tends to favor the buyer and any loss for the seller is to be viewed as a "cost of doing business." Conversely, Amazon does a bit more with eating some costs themselves if there is an issue. These are gross generalizations about these marketplaces, but in order for a marketplace to succeed it needs to be trusted and liked by BOTH buyers and sellers. Otherwise, each group will continue to look for alternate channels outside your marketplace. Let me know if you would like more insight, specifics or solutions.