Are you asking whether or not an oil tank is a legal requirement in the sale of a mobile home? Does the mobile home have an oil-burning heating unit? Or is it heated with a propane unit? Is there a heating system at all?
Whether you are buying just the mobile home (not the land) or a mobile home on land I recommend that you bring along a licensed home inspector who has experience in mobile homes. S/he will be able to help you analyze the heating/cooling and other systems of the mobile home. Also should a buried tank be an issue s/he may help you locate any tank that may be buried (or connect you with a reputable oil tank abatement company.) Licensed home inspectors can be found at the New York Department of State Division of Licensing Services: http://www.dos.ny.gov/licensing/homeinspect/hinspector.html
Best of luck!
I am going to assume you are really not asking if it is legal to sell it but what are the consequences with a buried oil tank? First you really do need to disclose to the buyer that there is a buried oil tank and it is the buyer's decision on what contingencies he may place in a contract with regard to the sale . He may decide to buy your property as is or may want you to certify, remove or abandon the tank. There is a property condition disclosure law that is pertinent to this situation.
The buyer may not even know what it means to have a buried oil tank and if the buyer is not getting a home inspector you'd want a buyer to be informed of the problems that may be encountered if the sale would proceed without any investigation.
Especially if the buyer is unfamiliar with the ramifications or unrepresented by a buyer's real estate agent you certainly would suggest seeking legal advice. Later on down the road should the buyer find a faulty tank he could perhaps seek remedy from you years later.
If there is a lender involved they will usually want to see that the tank has been certified or abandoned properly . A reclaiming service would be best to explain the procedure.
Good luck in your sale and always get legal representation.