Maybe he was undercover LE and maybe he was just some odd duckling who really had no clue; either way you did the right thing. We've had a few undercovers snoop around the gun shops around here before, and they're generally much as you described the gentleman in your account. Dresses the part, tries hard to fit in and join in conversation, but it's always a little off. Generally, they're easy to spot if you are a local. Never seen one attempt to initiate a straw sale around here (far less likely to fly in a small LGS than in a Walmart), they usually stay around until a transaction takes place and then they're in the wind and you never see them again. Of course, I can't prove they were LE, but you sometimes just know.
'Merica: Back to back World War Champs