Who will buy a Yahoo account here?
I think it's very easy to get a Yahoo account without having to buy someone else.
I don't see anything valuable.
Like what I said above there's some people that wants to get it with verification through SMS and others just want to claim it because of 'aged' accounts. Probably that's the reason why there are some sellers and buyers interested to it.
Nevertheless, if someone just want to have yahoo account they can easily register to it. You may not see it as something valuable but if its related to any business, I guess these people see some function for it.