I would assume he just mention that the SSD he is selling has Windows installed on it. He can erase the windows afterwards. We never know.The simplest way that doesn't require third-party tools:
Enable BitLocker (Windows) or FileVault (macOS) on the drive and let it finish the encryption process. Then remove the partition/volume and create a new, unencrypted one.
(But you really should check whether the Windows license allows you to distribute Windows installations like that - or, if nothing else, whether it's smart to tell the entire Internet that you're doing it...)