So, technically you aren’t liable for making any repairs that the buyer wants as nobody can “force” you to do anything with your property that you don’t want.However, if you want to sell your property to somebody obtaining a mortgage, somethings will have to be taken care of, eg a UST even if decommissioned.

If the township closed the oil tank pull without a dig, then there was no leakage, but since the tank (whether oil or septic, which is unlikely) was decommissioned and filled with sand that is not proper procedure and does not preclude a leak of oil.

This led the State agency to require a.) that there can be no disturbance of the soil at all within a certain footprint of where the tank was located b.) this defect has to be registered on the deed, c.) all neighboring properties have to be checked for active and passive water wells and if any water wells are not properly decommissioned (I believe this involves removing the hardware and plugging the well with concrete) they have to be tested now and over the next several years for contamination.

Just make sure you have something in there that says that the owner will remove (not just decommission but removr) any discovered inactive underground storage tank and remediate any contaminated soil if found.

So to ensure you comfort and safety you have decommissioned the central air unit and have provided 2 window air conditioners for their use but still your property.

I put an offer on a two-family home with a decommissioned underground oil tank.

@Michael S.Check if this holds in your area as available option, but in my area, as long as the tank has not leaked you can have it decommissioned and left in place. 

I have gone to the town building department to see if there is any record of a tank installation, removal or decommission (none), and have reviewed NJDEP data to see if there has been prior remediation on the property (none).  

@Carlos Ptriawan I was renting the entire building, which was a decommissioned storefront with an owner's unit above.

They removed the decommissioned oil tank, and remediated the termite and mold found in the basement.