Robbie, As you know, I'm just over the Clyde limit (well maybe, I've lost a bit since we last met a few months ago). I've ridden glued frames, and had no problems or concerns. I would have no concern about riding a vintage carbon frame because most of them where over-engineerd to assure they would not fail.
However, if your buyer is concerned about frame failure, let him move on and sell it to someone with confidence. I say that because if the dude has doubts, any problem with the bike could turn into an excuse to complain or return the bike.