Which of these fits the on your website or ad where you plan to place it?
This feels more conversational and approachable, often used in subtext or explanatory sentences. buy & sell direct online
Buy & Sell Direct Online
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. Which of these fits the on your website
Buy & sell direct online
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: "A direct-online marketplace")