What is the difference between Place an order and Quick transaction?
If you want to buy or sell digital currency immediately - select the Quick transaction option. Then you only indicate the amount you want to spend/receive, and the system will automatically use the best of the orders (at the most favourable exchange rate) and exchange funds for the digital currency you indicated.
Another method to buy or sell cryptocurrency is using "Place an order" option. After logging into your BitBay account under the graph, you'll find a "Place an order" option. To place a purchase order, enter the amount that interests you to the two of the three boxes (exchange rate, number of cryptocurrencies, or price). Value of the last, empty box will be calculated automatically. After clicking the "Buy" button, your order will be placed in the orderbook. You'll also find it in "My Orders" tab.
Post only - post only order is to ensure that you always pay the MAKER commission. When you try to set up this type of order, you can be sure that it will be issued to the list of orders (orderbook) or cancelled (in case of an immediate transaction, which would result in collecting the TAKER commission)