Okay so if you open the sell item menu, there's a couple things. First you can click a small little arrow to the right of the sell window, this shows you all current sell and buy orders. Next, in the left side of the sell window you can choose "Sell", "Buy", "Sell order" and "Buy order". Choose Sell order and the windows changes a bit and you can select the amount you want to put on a sell order and the price you'd like for them to sell at. Clicking the confirm button, you just created a sell order. Keep in mind it may take longer for it to sell (you will get a market notification in your mail box) but it will guarantee the price that you set, is the price that is paid. When buy orders are super close to the sell order price, I just sell them without sell order (but for me it has to be at least within 5% off each other)