Evidently, the legality of these things varies from area to area. Research that yourself, don't assume that the guy saying they're legal means they're legal. Anyway, if they're not legal to ride on the road, the value is just about zero. (Note also that some places have a limit on motor size, which is usually 50cc.) The once or twice I've heard one running, it had about all the charm of riding a bike with a chainsaw running. Loud and annoying, but not in a cool Harley way, in a dweebish model-airplane way.
To put it in perspective, I think I might have seen ONE of these around here one time. There's lots of people that want to sell you the motor, and a few that want to sell the bike with a motor on it, but practically no one actually using them locally.