Long ago selling accounts was not a big problem. People didn't think that high level accounts can be used for scams. Signature campaigns were not the business they are now either.
Back then there was even a website that was calculating the worth of an account. But no longer.

Selling accounts is now stupid - although allowed officially, seller, buyer, the account to be sold will be tagged when caught. And it's a good step forward, since the reasons for such a deal can be at least shady, if not plain scam.
Plus now the merit system makes leveling up difficult for spammers, so the way the worth was calculated in 2013-2015 (I don't know the exact years) is different from what it would mean now.

All in all, it's very difficult to know how much it worth. And unless you want to get a no collateral loan (and then the lender will assess the worth and risks) or sell your account for a few bucks to an 1xbit spammer (yikes!) I am not sure why would you care...