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.

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: buy & sell direct online