Your fellow players will not be the only ones to visit your settlement. The free traders will also pay their calls. These buy any surplus goods you may have to offer and sell goods you may need. They are convenient, but not always cheap. To do business with the free traders, change to info mode, and click your warehouse. You now have a choice between "BUY" and "SELL". If you want to sell something, click "SELL". Clicking one of the storerooms (empty blue squares) will show you an inventory of your products, where you can select the product you want to sell. Use one slider to set the selling price and the other to set the maximum amount you want to sell. Everything to the left of the setting mark will remain in the warehouse, while everything to the right of it will be sold. The info bar will also display the amount "set" in tons.
When buying ("BUY") goods the procedure is the same. The free traders will keep selling you the chosen product, up to the desired amount.