This scam originated in Las Vegas but it has since spread to other cities. Let's see how it works... You finally find a parking spot and are happy to see no signage telling you not to park there. You stop in a couple of places, run a few errands and return to see somebody waiting for you.

They tell you that you're parked on private property--their property. They may even have a boot on your wheel preventing you from going anywhere.

The landowner is very mad and says he'll only remove the lock if you pay him $100 on the spot--otherwise, he will take you to court and you'll lose a lot more. You are in a bit of a hurry and you certainly don't want to spend a day in court. What you should realize is that you're in one of two scenarios:
1. You really are on private property and you are the victim of cash extortion, which is illegal.
2. This man may not be the property owner, but a just scammer who “supervises” an empty lot.

How to avoid: ask for credentials. You should first establish that this person is the owner before you even respond to him. Consider calling the cops. With no signs telling you not to park there, he doesn't have much of a case--and he certainly doesn't have grounds to put your car in a vice. The other option, of course, is to park in an area you know is trustworthy--even if you do have to pay a couple bucks.