I like your analogy, except for one major difference: The UA GPU seller can easily be caught and punished severely by UA, without due process. (Viz. Guilty until proven innocent.)
All UA has to do is pose to be a buyer of a GPU, get the seller to call in with their MP account number. Then, optionally, check their records for additional GPU "gifts" by the perp.........Then bust him on the spot. Pretty risky venture for a few hundred bucks, wouldn't you say?
I don't think the Nigerian princes have much to lose and, in any case, would be difficult to prosecute and recover losses.
The points I'm making are:
1. To the innocent GPU buyer, he would probably think: "Well if it's illegal, UA can easily find out.......So, it's probably okay."
2. To the GPU seller, he probably has no idea of the extent of the damage incurred if he gets busted. Common sense would tell him not to do it if he knew.
It would be interesting to see how many GPU sellers would withdraw their ads if they knew the potential dire consequences of their actions! Hmmmmmmmmm, that would be easy to test!