Was this guy a little anal?
It might just be my area but, having classic yard owners that are a bit quirky (and not like the rest of normal society); is part of the fun.
I agree with just about everyone else here... it sucks but, classic car part theft is, and has been, an ongoing saga. Especially, for larger/outdoor yards. It’s hard to secure a whole lot of acreage from determined thieves and, I could see why the owner might not want their current inventory broadcasted to the interwebz.
Best way to earn their respect is by opening up your wallet each time you visit and, sharing your passion for older vehicles.