Legal Question in Criminal Law in California

First, call Nike to verify that this man did not work for them. He may be pulling a similar scam on other businesses; if so, Nike will want to know about it.

Assuming that he isn't a Nike rep (which seems like a safe assumption), stop payment on any other checks you gave him. Then call your local police and tell them what happened. The more information you can give them, the better the chances that they will find him.

Go file a police report of a crime,try to stop payment on the checks, and learn to conduct business properly with real manufacturers reps. Buying from someone peddling 'sale' 'discounted' goods out of the back of a van is almost guaranteed to be dealing in stolen property, for which you could be arrested. Someone peddling goods like you described would likely be a scam artist; either stealing your money, or by selling 'knock off' or 'gray market' or 'black market' products. Selling them will get you sued by the legitimate manufacturers.