Friedman is full of crap.

Buying or selling pressure are indication that someone thinks the market will move in a certain direction. They do not indicate which someone, why they believe that, or how confident they are in their prediction.

Thus, even if we assume all market actors are perfectly rational with respect to the information they possess (which is never true), a mere short-term price shift due to buying or selling activity, even when the quantity of the activity (units bought/sold and at what prices) is made public, does not actually give us information about the beliefs and confidences of the person buying/selling.

If I want to treat sales as information and use them to assess where I think prices are going to go, I have to have all information available about the sales. I can't even estimate other people's beliefs about the sales without knowing about what transactions they make and what their portfolio looks like.

Unless someone's got some kind of super-Aumannian probability math for applying to imperfectly rational agents of whom we have imperfect knowledge, which we can use to make reliable estimates of probability and financial value despite all those levels of imperfection.

EDIT: OW, my brain hurts, and I feel like I need to go eat an advanced probability textbook.