I have seen posts on this forum on occasion from people looking to buy the private keys to old addresses which used to contain large (by today's standards) amounts of bitcoin. The only reasons I can imagine that someone would want to buy such a private key would either be to lend credibility to a lie that they have been involved in bitcoin for longer than they really have, probably so they can attempt to scam someone ("I've been involved in bitcoin for years and handled hundreds of BTC, look at this signed message as proof!"), or so they can claim that those old transactions represent bitcoin that they have sold, probably for tax evasions purposes.