I realize you said you did not want to discuss fair rentals, but if " generally profitable" to strangers at a arms length transaction, and no personal use I would consider that fair market value. I would definitely not worry about properties that have been sold. I also would not amend any of them. In my opinion I would not file a 3115, but that would be the correct way. I have had several clients with rental property that are profitable, but have not raised rents as they don't want the hassle of new clients, and over the years those property's are falling below other property's in the area, but as I said if profitable, then that is market value. Just my opinion.