If I get interest in a contract I'm negotiating seller financing the amount owed on the mortgage subject to with no money down.

This way you have documented the event.There is an incredible story by Will Barnard who speaks of his 5-6 year experience when he purchased subject-to from an owner who had apparently had a relationship with someone who also lived in the property. 

BiggerPOcket has a book for sal "Tax Strategies for the Savvy real estate investor" with chapters on this subject

There is very little other reason to subject yourselves to having to deal with tenants.

Where is the subject property located?

If they wouldn't be subject to a gift tax, could they just sell it and then write me a check for $85,000?

true, but your asking for objective data to answer a subjective question.  

Right now, only lenders that make five or more loans annually are subject to licensure.

This subject is a loaded one but one I believe will help you based on what you are saying. 

Welcome Ramond,lease options,seller financing,subject-to deals are all really  great.