You can to login at several Dark Web marketplaces only with an links, what determines the following : inpending clients and vendors don’t have a possibility enroll until they will obtain an URLs from an previously enrolled vendor or buyer.
Those can be found at https://darkwebonion.com/, it stores updated link to all of the Deep Web marketplaces. You ought to follow these urls to register on the sites.
In case that there are some troubles with an access (links don't work), you may lie in wait and check for Deep Web markets alternative link. Carry on, as long as few marketplaces close and open signing up periodically, due to server load.