The subscription is as long as the person who owns the account continues paying for. If you have bought an account from me and your account stops working for some reason or too many people are watching at once, I will replace it completely free of charge.
I don't know if this is allowed or not. If the account is being actively used by someone and it's without their knowledge then it's stealing. I hope this isn't the case though.
A good question to ask would be if you could change the password to the account after purchase without problems from netflix or recovered accounts.
I don't see why it wouldn't be allowed considering Sythe is "the Craigslist of virtual goods". If you change the password after purchase you have almost a 100% chance of the account(s) getting taken back by the original owner.