How much "negotiating" did you do? Really, if you beat the kid down, there is not much chance that the seller will have much empathy.
If you paid top dollar; you might have a bit more chance.
I have sold a few bikes that grenaded right away. I never felt obligated to help fix them. I might have helped a couple of buyers, but only if they paid me what I was asking, and even then, it would need to be something that I might have had a bad feeling about.
If I "never saw it coming" then I sleep pretty good at night.
Oh, and by the way, the buyer had better ask nice, or his chances go way down.