Walk away if they dont want the tank removed. No one is going to buy that house with an inground oil tank..that's prob it's for sale by owner and as is.The home of interest is for sale by owner, as is where is. And it would not be a mortgage. All of that being said, I would have not protection. If I move forward I will either require the owner remove the tank and pay for an assessment or pay for an assessment and I will have the tank removed. The house is on municipal water but it has a well and so do the surrounding homes.