I want to buy a motorcycle from a private seller and he agrees to take payments. We both sign a contract stating that he will hold the bike until all

Assuming that the contract provides what you say it does, and that you have not breached it, no.  Even if you had breached it, while he may have the right to rescind the contract and sell the motorcycle to someone else, he would (unless the contract provided otherwise and maybe not even then) be required to return the payments you had already made.