You are allowed to log into some amount of Darknet marketplaces using only the links, that implies a tricky situation : prospective buyers and vendors are not allowed to sign up before they will gtab an URL from an previously signed in customer.
You can find those at https://darkwebonion.com/, it stores updated alternative link to described Dark Web marketplaces. You may follow these alternative URLs to register on the sites.
Whenever there are some problems with an access (links don't work), you ought to lie in wait and check for Darknet marketplaces urls. Keep trying, for the reason that some of the marketplaces shut down and launch registrations from time to time, depending on server load.