Hi A.B. If I were looking for a home to buy in your area, I would not consider a home with an underground heating oil tank. I think they are long-term hazards for contamination of your home, the basement, soil, etc. Oil is OUT, natural gas is in.
If I were you, I'd call my local gas utility and ask them how much credit they'd offer me if switch my service to natural gas. Many utility companies now offer incentives to switch to cleaner gas. Oil sometimes gives off foul odors and like you wrote, the tank is always at risk for a leak and possible contamination.
If you want t your home to SHINE when you put it up for sale, have the tank removed, switch to natural gas and do a soil test if you see signs of a leak where the tank was buried.
Scott Miller, Realty Associates, Boca Raton, FL
p.s. i removed an above-ground tank when i rehabbed homes in Philly. make sure you have a professional perform this work.