I would hesitate to offer replacement frags on high end corals when the mother colony is doing great. For all you know, the buyer put it in a tank that was less than 2 days old.
Its not like you are offering Green Bay Packers at $2 pp. Myself, I would want to understand how long they had been in the hobby, if they can fluently discuss water parameters about their tank, flow, lighting, what changes they made when the coral started to look "off", etc.
YMMV