Since no one replied, I'll put in what I know.
To answer your questions:
"Can anyone tell me is buying of YPN accounts allowed by Yahoo?"
Buying a YPN account is not OK. They are on to this and 9 times out of ten, after you purchase the account, YPN disables it. It cracks me up. I actually had a friend (not me) who tried purchasing an account, and it was disabled in less than 24 hours. And he was out a decent amount of cash.
I see in various forums people buying and selling the accounts. It just seems like snake oil to me.
"Do you have any information about it?"
If you google the subject enough, you'll find some hard data recorded by the poor souls who thought they could buy there way into YPN's rediculous excuse for adsense.
Long story short, save your money.