Verify ownership of a website

I'm creating a website where people can sell and buy digital products like websites, domains, scripts etc. My question is how can I determine if the user who wants to sell his website is really the owner of the website and not just an administrator or a developer who has access to the files? If anybody can give my an answer on this and a reference of technology to use to verify the ownership would be really great. I'm using laravel 5.1 for this project. Thanks in advance

via Chebli Mohamed