Not a "house-flipper", save for the sense that I would happily pay to move from a plot I have, to a plot I want. Like I said, my personal "end-game" goal would be to have a plot I like, in the same Ward as my fam and friends. I already have a pretty nice plot, but I'm really hoping whenever Ishgard opens up, to be able to get a cool plot there, instead. There's absolutely no way I would profit from the system, as I'm the one who's asking for a way to BE ABLE to spend gil to trade for a plot I want.
Now in theory, yes, house-flippers may still exist. But only so long as they have a desirable plot. If no one wants to trade plots with them, then that plot is effectively useless.
You could couple this idea with removing the priority-access that relocations gets; if you want to relocate to an empty plot, you should have to use the same Lottery system as everyone else will, however that winds up working out.
I recommend adding a tax there, because for players to want to trade plots, it would obviously involve trading gil as well. As well it SHOULD. But with that hefty 20% tax, that gil just disappears from the economy, meaning that over time, this would level out, at least more so than they currently are.
There will always be players looking for ways to get gil. I got a lot of gil but just dedicating some time to crafting items and selling them. With this system in place, even if you wind up getting a plot you aren't super crazy about, it would give you a means of saying "Alright, I'm gonna spend some time on my tradeskills, get some gil, and then working on getting a plot I REALLY want!"
And like I said, by no means am I suggesting this in lieu of adding more Wards to the game. I would love to see more people able to get houses! All I'm saying is, give us a means to trade our way up, from a Plot we aren't crazy about, to one we can be super happy with!