SE already made a change where 'purchasing a plot with intent to sell' has become a prohibited activity.

That means that purchasing from the placard (or even possible another player) and moving directly to looking for a buyer is already disallowed. Ostensibly, it is still permitted to eventually sell for gil an FC that was originally purchased for use as a social FC, a workshop fc, or part of a decorator's collection. This can be a very good thing, as it allows players who have put time and effort into their FCs to reclaim some gil for those efforts before transferring servers.

"Cash site" sales as you mention are already very much illegal. The only real life currency you should ever redeem playing ff14 should be via mog station and your game subscription.