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
Using the symbol is generally better for digital layouts because it saves space and creates a visual "anchor" between the two main actions. If the tone is very formal, use the word "and." 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: You do not need a hyphen between "Direct"
This feels more conversational and approachable, often used in subtext or explanatory sentences. Here are the three standard "proper" formats: This
Which of these fits the on your website or ad where you plan to place it?