What do I expect GMs to do? It wouldn't be hard to implement a mechanism in the game that triggers when someone relinquishes a plot AND receives a large amount of gil at the same time.

Now that I think about it, relinquishing a plot is a BIG DEAL, considering how sought-after plots are. Plus, there aren't that many plots per server. Think about how often plots get relinquished. I'd say 10 a month would already be a big number. Now what if whenever a plot gets relinquished, GMs (or equivalent) get notified and are able to investigate the character who did so, to try and see if it was sold to another player? It wouldn't take a lot of time, and it would turn the spotlight into people who only bought a house so that they could sell it.

But honestly though, with the way it gets handled, it's like this sort of thing isn't even against the rules.