You can to log into certain Darknet markets using only the link, which determines a tricky situation : upcoming vendees and dealers are not allowed to sign up before they will get an alternative URL from an previously enlisted user.
Look at http://deeponionmarkets.com/, it posts active alternative URL to suggested Deep Web marketplaces. You ought to follow these URLs in order to get access to the sites.
If there are some problems with an access (links don't work), you should lie in wait and check for Darknet markets alternative URL. Keep trying, because a several amount of the markets close and launch signing up from time to time, due to server load.