All squad members have lockers in the Normandy's cargo hold. These allow you to change their equipment. To change a squad member's equipment, approach their locker and press [ E ] to open the equipment screen.
The Requisition Officer is a vendor available in the Normandy's cargo bay. You can sell unneeded items to him for credits. He also sells a variety of equipment. His selection of wares is initially quite limited, but expands as you discover additional Manufacturing Licenses.
When shopping from vendors across the galaxy, you will find that some offer license for specific manufacturers. For example, you may purchase a license to manufacture Elkoss Combine equipment from a volus vendor. This would add equipment manufactured by Elkoss to the Requisition Officer's store inventory.
The items available from the Requisition Officer change every time you visit a new plot world.