leakdown test, heat cycles and break-in before ever riding it
and........i would either make it right, or tell the guy, before he trades you, what the problem is, he now owns a bike with an airleak, and it is gonna fry real soon, if he even fires up that engine again before fixing it
or he'll be back with your bike fried, wanting his trade back
or, let us know on here if you trade that fugged up, as it is now
so we know not to deal with you on here