As an unlicensed person, you may sell to a FFL in any state. FTF sales can only be made to a fellow resident of your home state. If you are in MD, and buyer is from VA or PA, NO.
You are permitted under Federal law to make "casual sales", such as disposing of a collection. That is not considering "engaging in the business".
Not that you asked, but would do a 2 copy bill of sale, your name and address, THEIR name and address, description of gun. You BOTH sign, each gets a copy.
What we have here is... failure- to communicate.