Well, that's how we run bitcoin. We sell and buy and sell again and buy again. What matter most here is the strategy, and selling btc is not acceptable to be called as stupidity. However, it is important to be wiser in different strategies of how you will earn from bitcoin, whether you sell it, as long as you will not find regret at the end, cause if you will end up with regrets, well that's what you can call stupid.