I think a forced auction after a lengthy period of inactivity is a good system, and the sales proceeds are credited to the player's account so they are not out of pocket. Have the starting bid at the base value of that plot, since for small plots people are willing to pay a lot more and the increased value reflects inflation.

Really just being able to sell houses directly would reduce a lot of demand in general, there are a lot of virtually dead FCs that own prime real estate and their surviving members would probably like money more than the house.