We currently are held responsible for every public thing happening on this website and server. Although sellers should be safe, the risk of getting scammed as a buyer is rather high. It would take a whole lot of legal policies to get this done.
Please use the '
Broadcast Yourself' forum to advertise the products you would like to sell or trade. In here, you could also place payment buttons to whichever website you like, including but not limited to PayPal. As long as it is situated externally or on a private basis like through PMs it is all fine.