The most professional and visually balanced way to write that phrase depends on how you intend to use it. Here are the three standard "proper" formats:
You do not need a hyphen between "Direct" and "Online" unless they are modifying a noun (e.g., "A direct-online marketplace"), but for a general call to action, keep them separate. buy & sell direct online
This is the standard for marketing headers. It makes each word feel important and intentional. The most professional and visually balanced way to
Which of these fits the on your website or ad where you plan to place it? "A direct-online marketplace")
Buy & sell direct online
Buy & Sell Direct Online