Black Market in XCOM 2 is a special vendor where you can acquire useful resources.
Black Market Information
Through the resistance you gain access to this shop during the progression of the game. The Black Market gives you an opportunity to sell excess items and equipment in exchange for Supplies as well as exchange Intel for various items such as resources, Upgrades, Personal Combat Sims, Scientists and Engineers. You may also at times purchase a boost to the speed of projects.
The items for sale refresh every time the Black Market is available. The Black Market reappears roughly monthly on a random day, making it a resource you will want to check when it becomes available for its updated inventory. Just be mindful of the amount of Intel you spend at the market, as Intel is a valuable resource for expanding contact with new Locations and countering Dark Events.