Is there a chance that the person who is buying such an address is trying to fool some upcoming user into showing that they control a certain "old" address and hence attempt to gain the trust of the person in front?
This used to be the case with most of the accounts being sold in this forum, I am trying to project a similarity.
Airdrops, like @GreatArkansas said, could be a reason but I dont see the ROI happening from those airdropped coins vs the amount spent to buy that address. The above scamming methods seems a better investment, logically. After all, which airdrop made its receivers a millionaire? None.
Cryptocurrency industry is magical, there are ways we can make money that other people can't think of.
If what I suspect is true, then it is a scam not magic. I would suggest the OP to not deal in such things unless they are sure of the buyer's motives.