It’s not allowed if there is trading of real money involved (which there usually is, but the people advertising are smart enough never to mention that in-game). However, since the PFs do not blatantly advertise this, the GMs cannot do anything about it. Only way they could do something about it is if they either accept screenshots as evidence that the sellers start asking for real money once you contact them in Discord—this would require them to loosen their restrictions on accepted evidence of ToS violations—or by setting up sting operations—this would require more manpower in general. Or if the seller wasn’t smart enough and mention “We want $500 USD for Ultimate clears; PayPal only” in-game (because the only evidence SE accepts right now has to be present in in-game logs).