I don't have one, but thought about buying one.
There are scads of them at the racetracks I go to, running around the infield.
I think your buddy missed the window of opportunity.
There was a huge boom of these things for a while, with everybody in the world selling them. (Gas stations, anybody) You can go on ebay and buy a container of them for under $100 each. The Chinese knockoffs have flooded the market, making it difficult to sell the better models made by Blata and Polini....which go for 3-4 times the price of the chinese stuff.
A lot of kids have ended up getting hurt or worse on them, (Due to STUPID PARENTS!!!!) as well as dumb adults. A lot of areas are starting to prohibit them on any public property....streets, parks, etc.
As far as selling parts, I'm not sure what type of parts he wants to sell.
I think there are probably a handful of chinese made engines being used, and maybe some wheels, tires, and stuff are universal, but not sure about the frames.
I'd have him look at ebay, and see if this is something he wants to dive into.
A buddy has one of the Blatas. He rides it occasionally.....