It's pretty straightforward. For simplicity, we have Players A, B, C, D and E.
- Player A has Master status in eight Rank 6+ FCs on their eight characters. Players B, C and D have characters in each of those FCs. This satisfies the 4-person requirement for FC house eligibility, and any additional caveats (4 characters on unique service IDs).
- Player A purchases Plot 1. Players A, B, C and D are now no longer eligible to purchase FC houses.
- Player A kicks Players B, C and D from FC-1. Players B, C and D are now eligible to purchase FC houses.
- Player A invites Player E to FC-1. Player A gives Master status to Player E. Player A leaves FC-1 and is now eligible to purchase FC houses.
- Player E now owns a single-character FC and is no longer eligible to purchase FC houses. Because no checks will be done on grandfathered estates to remove them, and because checks would punish any FC that drops below 4 members for any length of time (even 3 very active people), it is assumed no checks will be done on Player E.
- Player A-2 purchases Plot 2. Players A, B, C and D are now no longer eligible to purchase FC houses.
- Player A-2 kicks Players B-2, C-2 and D-2 from FC-2.
- Player A-2 invites Player E-2 to their FC. Player A-2 gives Master status to Player E-2. Player A-2 leaves their FC.
- Player E-2 now owns 2 FCs with houses and remains ineligible to purchase FC houses.
Just continue that until Player E owns 8 FC houses. All purchased post-4.2. All legitimately, all largely uncontested.
There are ways for SE to combat this. However, those ways will have a dire impact on non-resellers and/or put additional strain on the server by virtue of constantly refreshing to check eligibility status on either an account or instance level. It is far more likely that the only check in place will happen at time of purchase, and as shown, it's hilariously simple to bypass that.
If SE does implement a series of checks for this having spent 3 or so years blatantly ignoring hackers, amongst other things, I might implode.