how do you keep people from stealing them when you have to leave them unattended?

We never really leave them unattended. Always park them within view and keep an eye on things. Plus I've done things so they don't simply start when parked, they would have to be pulled on a hook if someone wanted it, and that draws attention.

Helps that we don't live in a real high crime area either. Primarily retired folk up here, and the town is full of classic cars seen on a pretty regular basis.

There's a reason quite a few large collections have been selling the last 5-7 years. Asking and selling price are often not close. When you see a particular car go for eye popping $$ at auction it's a high water anamloly that we all love seeing, but most likely two krypto millionaires with alcohol going at it. Agree that the pedigree cars will always command proper valuation, but as stated the overwhelming majority of young people see cars as an inconvenience, and categorically do not have the passion nor interest like our generation does. Personally i'm going to start hoarding early smart phones as those will be en vogue to millennials in a few years. They will be the bandit T/A's of that generation IMO