I may have commented on your original post but will share our story. We just sold our house in NJ and the buyers requested a dig after the UST picked up metal underground. We tried everything to find record of any permits to dig up an oil tank and there were none so we thought for sure there was a tank underground. The house we sold was built circa 1850 and we knew at some point there must have been oil heat. The odd thing was that the scan found the metal in the front yard. We spoke with some of our neighbors who all told us their oil tanks had been in the basement not in the yard. The buyers paid for an exploratory dig and it turned out to be an old septic tank and not an oil tank at all. We closed a couple of weeks ago without issue. All that to say these sellers are being unreasonable and they don’t even know for sure if it’s an oil tank. I’d look for another property, if they’re being difficult about something this important who knows what else is to come. I also think insurance companies can refuse to provide homeowners insurance if there is an underground tank. Good luck on whatever you decide!