Lack of housing plots are a huge factor in the buying / selling of Gil.

[Sicno]

House prices really aren't that bad if you're a crafter, sell carries or buy/sell "hot" items. Also a group of crafters/sellers (a FC) can easily raise enough gil for a house(even to buy it from another player). I don't know anyone who buys gil and I doubt it's as common as you think. I'm sure once in awhile a lazy player buys gil for housing... but if they're lazy then they'd probably do it for all kinds of reasons, not just housing.

That applies for normally buying a house. OP is talking about house re-selling, which happens at exhorbitant prices because there are normally no available plots, they're way more expensive than the regular prices (I saw someone at Adamantoise trying to sell a small house for about 8-10M in party finder). And then the buyer has to pay the real price of the plot on top of it.

But I agree with Blood-Aki, lazy people are gonna buy gil just because they're lazy and they want as much help as can get handed to them. Housing is just one of the many things they must spend their money on, it's not the core of the problem.