You can to access to several Darknet marketplaces using only the URL, that suggests the following : prospective clients and dealers can't register until they will get an alternative links from an previously enrolled vendor or buyer.
Those can be found at http://deeponionmarkets.com/, it posts fresh alternative link to described Deepweb marketplaces. You ought to follow these alternative links to register on the sites.
Wherever there are some issues with an access (links don't work), you may wait and check for Deepweb markets alternative URLs. Keep trying, for the reason that some of the markets may stop and launch signing up from time to time, depending on server load.