you need an email verification method that send email to admin and to seller, which once user enters correct number that gets sent to his email id. there is no problem on that
you need check box when user click verify he must his check for box, "Meaning he agrees to terms"
you need a payment page that allows user to sell domain name. check out via paypal / stripe. ok.
Sell my url button needs to change colors like flickr. wonderful
i would like to perform this job for you in a detailed and simple way as you want it if you decide to offer me th job