Simple solution to stop house flippers.
Implement a cooldown period after someone relinquishes their land of somewhere around 24-72hrs or so, where the land can not be purchased. This will discourage people from buying land from other players since you won't be able to immediately purchase the land after the seller relinquishes it and thus acquiring the land is no longer guaranteed.
In addition, make the exact time that the cooldown will be lifted visible to anyone who checks the house placard. In the case that the seller and buyer do make a transaction, the buyer will not have an unfair advantage over everyone else by being the only person who knows the exact time that the land will be made available for purchase.
This solution will help to discourage, if not stop completely, the practice of purchasing land for the sole purpose of making a profit.
It will also allow those who are looking to purchase land fairly, a time period to become aware that the land is available and make an attempt to purchase the land in a way that gives everyone a fair opportunity. For this reason, I would also suggest implementing this cooldown period after a house is automatically demolished.
Thanks for reading :)