What could be the reason behind people having to buy old wallets and could there be any risk attached?
It's usually because of airdrops. Some projects make airdrops for active addresses. The only risk I could think of is if your address is somehow attached to your identity (maybe you linked it by signing a message or something of that nature) then that could backfire.
If the purpose was airdrops then the people who buy the old cryptocurrency wallet will have the specific wallet they are going to buy. An example is the UNI airdrop which was given to all the self-custodial wallets that execute crypto swaps on Uniswap.
Even if we ignore the area of identity. I think selling of old wallet is a naive decision for the seller because no one in his right mind will dare to pay a certain fund for an old crypto wallet if it isn't worth more.