I wouldn't be rushing into selling him because of this. All horses can buck, if there's discomfort or stress.
You say he hasn't had spurs on him before, he's young, he's a sensitive tb. Yep, they can and will buck if you shock them with the spur. And sore in the hind end, so he's being spurred, he's sore behind - I know I'd try to buck someone off if I was in that position ;)
Get that soreness fixed before you do anything. If you want to spur train him, find a good rider that won't get flung off and knows how to control a buck, and use roller spurs not dummys or rowels.
THEN make your assessment on selling him. Right now, if you advertise him, you MUST tell potential buyers that he recent bucked someone off, and that he's sore. I wouldn't want to sell a horse like that, personally. Fix the problem, then look at selling if you still have to.