The Rainbow Six Siege Marketplace is where you need to go whether you're looking to exchange some unwanted items for R6S credits or get that elusive weapon skin from a few seasons back.
Browse for items under Buy or pick an item type from the Marketplace homepage. Only currently tradable items are available. For example, items from the current season won't be tradable until the next season comes out.
To purchase an item, put in the highest price you want to pay, create an order, and wait for a matching sale. The price you set is used as a maximum as the Marketplace actively seeks the lowest price. The transaction completes as soon a matching seller is found.