A conflict of interest accusation is pretty serious, and I think if all you had to go by is your skepticism, you probably shouldn't just make that claim as if it were a known fact: that's just very dishonest.

Sure, be skeptical, but then maybe say something like "I have no evidence for this, but I believe his manifest enthusiasm can only be explained by the fact that he's trying to manipulate the stock price to his own benefit."

Sure, it sounds less rhetorically convenient, but it's more honest.