Hard drives automatically remap bad sectors, so even if you overwrite 100% of the accessible data, you may still have left pieces of data in remapped sectors that weren't overwritten. If your drive supports the SECURITY ERASE feature, then it should overwrite those blocks too.
Though if you really want to sell or give your drive to someone else, the best thing to do is to use full disk encryption from the beginning, then there will be no plain text data on the drive.
Though if you really want to sell or give your drive to someone else, the best thing to do is to use full disk encryption from the beginning, then there will be no plain text data on the drive.