Insider trading involves trading that violates a fiduciary responsibility. That is: you work for a company owned by shareholders, and you trade based on information you have as a consequence of that employment, harming the shareholders.

Without that fiduciary responsibility, it's not insider trading, it's just trading. If you overhear someone with inside information talking about some inside knowledge, then trade based on that, you're in the clear (as long as you didn't collude with the leaker.)