Buying back shares knowing they're massively overvalued would be destroying shareholder value. Buying them back when you believe they're undervalued creates shareholder value.

If that's all true, selling shares when the market is over-valuing them seems like it's creating shareholder value to me. (It's unethical perhaps; perhaps it's illegal. Either of those would be reasons to think it's harming shareholder value, but the sale itself is creating value.)