Not your fault. If he wanted to sell out to some other people and cant because no I/O panel, that's his oversight.

You never mentioned anything about I/O panel and he didn't requested for one either when trade was made, so you're only responsible for the goods you sold.

If you really want to help, as they say can go Pc shop ask for I.O panel then send to him. If not, tell him that and ask him go get himself.