Great point:
I like this approach very much. I think the basic fee for participating as a seller in a peer network should be as low as possible. This allows the marketplace to develop as much liquidity as possible. Increasing transaction fees will push sellers out of your market into other ones. The better approach to increasing revenues is value added services that sellers can avail themselves of but are not required to. If these services allow sellers to sell more or if they make selling easier, sellers will adopt them and your take rate can ultimately be much larger than your transaction fee.