I bought one but it is a new google account.
The seller needs your current google account login details to attach the number to your own account but I feel it is too risky to give out your google account password.
Yes, Without login no one can add number to your old account...
And if you feel risky then there is an option....i will help you to add number...you can add me on skype to chat. Skype : ami.himu5