To be pedantic: the market is perfectly valued at every moment. Of course market prices will change, sometimes very quickly.
BTW, this is a great site for tracking S&P ratios: http://www.multpl.com/
BTW, this is a great site for tracking S&P ratios: http://www.multpl.com/
Practically speaking, I personally don't believe the efficient market hypothesis due to the very obvious meddling by political actors, central banks, national treasuries, etc. Behavioral finance has also consistently demonstrated that humans do not react rationally in markets.
My point is a pedantic one referring to the OP using the word "overvalued". The market price is never over/under valued. It is merely the market price at that moment.
As you said, it is merely tautological.
In other words I think it's futile to try to attach moral attributes (for example saying that a market is "best" valued at the moment or not) to things like financial transactions, which don't have any intrinsic moral values associated with them. Otherwise we risk talking about long-dead German philosophers when trying to illuminate the problem.