Bill Gates sells Microsoft shares

I don't see the similarity between your comment and the point I was making.

When a very successful long term investor buys a long term investment it can be a notable signal of the merit of the investment.

But if anyone sells an investment (absent of a credible explanation why) it doesn't say anything about the merit of the investment.

Further, if insiders (board, execs) buy shares in the company they are running that's a positive signal. It the majority of insiders sell shares that is a bearish signal. If one important insider (e.g. CEO, founder, etc) sells shares, that on its own doesn't really mean anything, because one person/entity could be selling shares for any of many different reasons.

One more nuance... When warren buffet sold off some airlines, and then later I think he sold Morgan Stanley or some other bank stock, and he explained in shareholder meeting (and probably some reports) why he sold these (unsustainable business practices) that is a valid bear signal.

So really it's when someone sells, and they don't explain why they sold, or their explanation has a conflict of interest, then the sale is not a meaningful signal.