My concern is that I know the child. If he was physically too big for a 5pt harness it would be less of a concern but he would be flat out meeting the minimum 14kg much less outgrown a baby seat. The trouble is, the seat she bought was a short seat, only rearfaced to 9kg but she is poor so nothing could be done. But there are inexpensive 8 to 18kg seats on the market.
I was thinking about this today.... Who called you a troublemaker? The potential buyer or the seller? If it was the seller perhaps you could pm the buyer to inform them of the legalities surrounding car restraints. If it was the buyer though, mmmm I'm not there's much else you could do.