Skill: Guild Merchant

Page 132 of the PHB states, "You learned your skills as an apprentice to a master artisan, under the sponsorship of your guild, until you became a master in your own right".

While that simplifies the rules concerning backgrounds (simplicity is a goal of the 5e system), it doesn't leave much room for character growth. As such, when you take Guild Artisan as your background you graduate at a slightly lower level. Replace the above with,

You learned your skills as an apprentice to a master artisan under the sponsorship of your guild. At the start of the game, you have reached journeyman status, which means you are free to go out into the world and practice your artisan skills. Journeymen are expected to wander, so you have a set of portable tools, but lack a shop or workspace. Through additional training, experience, and financial development, it is possible to advance to Master and even Grandmaster status. Such titles must be awarded by your guild.

Speak to the DM for further clarification of your specific background.