You go to your deactivated inventory. Then you click the item you want to put on market. There will be a place where you can put a price. Make a reasonable price for the item and click the "update" button. Then it's on market. But remember to check the market prices first. If you put something on market for 2k, when others has it on market for 1k, then you won't get it sold. You can find the link to market right under the big picture with the green lama and the pony with a horn.
PM me if you don't understand.