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I suspect you would be in trouble threatening lethal force for nothing more than blocking you in. No threats, no apparent weapon, daylight -- stay alert, certainly, but drawing or presenting your firearm would probably not go well.

I've heard of this type of approach before (selling out of their trunk). Usually stated that it's excess merchandise, and they'll give you a good deal. Mostly it's either fraud (you get a box supposedly containing a TV, that contains bricks instead), or the merchandise is stolen. Best to do just what you did -- leave, and call the police.