Nobody can steal your domain, even if it is unlocked. You might have been scammed by the company that sent you the email asking if you wanted to sell. Did you respond to the Godaddy email about unlocking? If you hadn't unlocked it this is a red flag. Are you sure the 15% offer was from Godaddy? Some unscrupulous companies send bogus offers, pretending to offer renewal, when what they are doing is asking for authorisation to transfer your domain to them. The internet is full of thieves. Always check your domains regularly to ensure they are up to date and secure. Godaddy is a reliable provider. It wouldn't be their fault if your domain was transferred fraudulently, you would have to have done something to release it.