Either way, you are responsible for making sure your customers are fully aware of what they are purchasing any hidden information needs to be said.
Therefore, you are responsible to refund 100% of what was paid for the account. Why? Because the account has been played on for a month and giving it back to you would be quite ridiculous because it is more than the value in which you sold it at. If you do not refund, the amount specified you will be banned.
Also, to prevent future situations like this I recommend making your customers aware of the account you are selling them prior to accepting their money.
Decisions has been made based on
@Charlotte,
@Gen and
@video's opinion.