Market Stand allows you to sell resources you have and get some Green Bucks in exchange. Each subsequent upgrade of the Stand gives you more crates to use for selling stuff.
To put an item in a crate, click “Add”, choose an item through tabs (Harvest, Production, Resources), and choose the amount you’d like to sell. You’ll see how much you’ll get for your things. If it suits you, click “For Sale”.
It takes 3 hours to sell items in a crate, once the timer reaches zero, you will receive your money. Of course, you can always spend some Gold Coins to speed things up.