I do think the anonymity of forum posting makes this a bit of a better fit for Facebook. Stuff like this sometimes happens in the
DCM forum, but there people are at least committed in the sense that they've donated to the site.
Back before Paypal, the Swap Shop really felt like the wild wild west
. Given that the payments would just be going to you through Paypal, I'm not sure about scam potential (maybe someone claiming they didn't receive the item - but that seems like a stretch with Paypal's controls now, if you were documenting things well). But still might be a better fit for Facebook where you've got at least a little more accountability.