I don't agree with tax system at all. i wouldn't be against a penalty system for the purpose of vacating lots. Everyday your house goes unused by you or your FC you get charged $xx amount of gil for the "upkeep' of that property in your absence. once the cost in penalties exceeds a certain percentage of your housing value the house becomes forfeited and returns to the market.

Also the gil sink theory, while applicable in the economic understanding between rich and poor players doesn't hold much weight in the context of absent players as any money in hands of a player who is absent has zero purchase power and is essentially 100% out of the market unless the player returns at some point.