Create a sell/buy/trade website. The catch is, there are not only first degree negotiations. Sometimes, person X is selling object 1, and would trade it (with or without money difference) for object 2, that person Y owns. But, person Y would trade it for object 3, which person Z owns. Gladly, person Z would trade object 3 for object 1. So it's a triangular negotiation with trade. It has to have a payment service built in. At first I thought of it for cars market, but it can work with other stuff too. Pre-written proposals will be dismissed.