Even if the Seller provides paperwork showing the oil tank was properly decommissioned, that is no guarantee that there has been no contamination of the surrounding soil since then, or that the integrity of the tank will not be compromised in the future.
The most prudent course of action is to have the tank removed prior to closing. As part of the removal process, a municipal inspector should be present to assess the condition of the tank, and to determine if soil testing is required. You may also choose to have the soil tested after the tank is removed, regardless of whether the inspector requires it.
If contamination is found, the Seller should pay for remediation and obtain a “No Further Action Letter” from the New Jersey Department of Environmental Protection.