All right, here is the PBS Team word on this.
You can sell your PBS Credits here. 
You cannot sell anything else.
That means that [name removed], you could have posted "Anyone want to sell me some credits? Contact me by PM if so," and then in Personal Messages explained what you are willing to pay. But you should not post a topic selling anything for credits. If in PM negotiations anyone was willing to accept the currency of another site for PBS credits, then you could make the exchange.
It is a small point, but there you have it. Don't hawk your wares (whatever they are) for credits. But you can ask if anyone is looking to sell credits, and negotiate payment with them privately.
We are sure someone else will find a way to do the above inappropriately somehow, and we will hear from members again asking us to clarify, but for now this is the ruling.