This is from Beth and it sounds like it comes from the top:
I'm glad you asked and we investigated! As it happens, you cannot post an offer like that. The loophole that's been left is that you can say something like:
"Is anyone selling PBS Credits? Please PM me"
Then in the PM you can offer your other site credits, you cannot mention them, or even the 'what' that you are offering, in your post.
So here goes:
Is anyone selling PBS credits: Please PM me. (No $s involved.)