The issue here lies with inheritance, which ultimately is a problem that cannot be solved due to how FCs *need* that feature to pass off the Master role in the event the lead wants to quit the game or goes inactive and doesn't bother to pass off the role beforehand.
The system works properly, if you bought a plot on a character you cannot buy another one on the same character without leaving the FC. However, if you pass the role off to an alt, you can go buy another house without an issue. This is also literally in their guidelines for the Additional Plots and Purchasing Requirements so it's something that is fully acknowledged already by Square.
I'm very very very positive by now that SE is aware that people multi-own but they also can't really do anything without changing the system that will affect *everyone*, not just them. They also most likely view it as a positive since they're making extra sub income off those who do purchase houses on multiple accounts.
I am firmly against RMT in any form but I personally don't really care if someone wants to sell their house for gil because they want to move servers or dump their houses and free up a service account so they can cancel it.
As I've parroted on many threads of this nature over the past several years, I feel for the people on servers like Balmung who have to deal with this (where you are basically lucky to even get an apartment, let alone a house). With the addition of newer servers and the rising complaints from members of those servers, I feel that the conversation is becoming more "There's no housing on my server I'm interested in even though there are dozens of plots open in each ward." If the latter is the case, I understand the perspective but at the same time, just get a house and relocate later. The point is to have one and if you're upset about multi-owners, buy a house so it's one less that they can buy lol.
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Someone I once knew, who is/was probably one of the biggest multi-owners in the game, said something that kinda stuck with me: "I bought my houses when no one else was willing to move and buy them. They sat alone in my ward abandoned for months so I slowly picked them up when it became obvious that no one was making moves to purchase. If people wanted the houses, they had plenty of time to get them."