Ride it over to the seller's house, ask him to remove it and also ask him for his grip-strength exercise regimen. Alternatively, warm up the engine, squirt some penetrant around the cap, shut down the engine, tappity-tap lightly on the top of the cap with a small hammer, go make a spinach and tomato crêpe, return with a long-handled channel lock or adjustable wrench then do counterclockwise what comes naturally.
You can always buy another cover if things don't go your way. Be sure to request proof that the cap is removable on that one.
