any way to remove J-B weld?

Yea, whatever is under there is either going to be time consuming, expensive, or both.
leave it the hell alone & politely tell your buyer it is what it is. If you feel its appropriate
maybe work out a price adj. , otherwise take the bike back if he's not ok w/ that.
You didn't say what type of bike, modern/plentiful or older/vintage, so I guess that
could dictate your decision more. unfortunately, casing & flying debris is part & parcel
with any trail/MX bike that actually gets used as intended, so ........ Kudos for
not just telling him to scratch his sac, but you can only do so much now that your here.
Good luck!!