It's not just GPSs. It's all kinds of popular and expensive electronics. The first giveaway is the price, the second is they will almost always have an email address they insist you write to. People give them money every day though, unfortunately.
FYI - as a real minor theft deterrent I erased the stock Garmin "splash screen" that comes up when you turn the GPS on. Mine has a Cessna motif (looks like something more "stock" than not), and on the bottom it has my name and phone number. Resetting the GPS won't erase that screen, it has to be reprogrammed.
