My 17 year old sold dirt bike without permission... legal?

My 17 year old was upset with his dad and out of spite, sold his dirt bike (worth about $600) to his friends dad for $150. I called the dad the next day, explained the situation and he said no problem about buying it back. Next time my son was in the area (they live in a different town) have him call and they would meet up. My son works alot of evenings and this dad gets home in the evening so it's been 1 week. We finally talked to him today and he said that he needed the money and sold it. (word is for $300)
He has our phone numbers. And my son has been trying to get ahold of his friends all week. With no one calling back.
Since he's only 17 was that legal? What am I supposed to do?

You stated that he "sold his dirt bike..." essentially admitting that he (your son) was the de facto owner. Your only probable recourse (dependent upon the laws where you live) would be to say that he wasn't...and press charges against him for stealing and trafficking in stolen goods. It sounds like the friend's dad acted unethically, but if you didn't represent to him that the merchandise was stolen (thereby injuncting him from reselling the goods), he didn't act illegally.

Good luck...

Was there a title for the bike? Was it in your sons name? These questions could make a difference. If no title changed hands, tell the buyer he has a certain number of days to return it before you report it as stolen. Personally, I would charge your son the money for being stupid. He obviously has anger problems.

since he was not 18, he cannot make a contract and the sale is not valid.

You either have to agree on an amount he should pay you, or go to small claims