You are allowed to log into several Dark Web marketplaces using only the alternative link, what implies next : prospective clients and merchants can't sign up until they will acquire an alternative URL from an already signed in customer.
Look at https://darkwebonion.com/, it stores active urls to described Darknet marketplaces. You ought to go to these urls in order to sign up at the sites.
Wherever there are some problems with an access (links don't work), you shall lie in wait and check for Deep Web marketplaces urls. Keep trying, for the reason that some of the marketplaces close and launch signing up periodically, due to server load.