Merchants for Mass Effect are Npcs that can be interacted with to obtain goods. To purchase from them players need to have a certain amount of credits, the equivalent of money in the Mass Effect universe. Each merchant also has a large inventory of randomly generated items and some specific products. Unlock conditions for merchandise are generally two types. Some are event-specific, for example when Command Shepard becomes a Spectre and new gear becomes available. The majority of remaining unlock conditions are level-specific and become available as the squad gains experience throughout the game.
The more manufacture licenses you buy during your career, the wider the variety of goods available in the comfort of your own ship will be, which effectively reduces travel time when you need gear upgrades. A license only needs to be bought once for each manufacturer and you get permanent access to all their products.
Mass Effect Merchants