I don't see how this makes the economy inefficient. Could you elaborate? And in this particular analogy, the purchasing decision was clearly the buyer's, so the worst you could say about the seller was that he was pushy and obnoxious. I can't see why one would 'blame' him for the inefficient economy.

Statistics can say that with over 99% confidence.

That's disingenuous. What I meant was, based on the story of the pushy piano purveyor alone, without knowing what use was made of the instrument, you can't really say it was a useless purchase. Serendipity happens.

Deals are supposed to create value. For example, if a house is worth $2M to the seller and $3M to the buyer, there's $1M potential surplus to be captured. A deal at $2.5M makes that makes that surplus real, in effect giving $500K to each. If a potential deal doesn't create net value - if the buyer doesn't receive enough benefit from the good, or the seller spends too much manufacturing it - the total wealth in the economy goes down as a result, even though the buyer OR the seller might win personally.

Serendipity happens.

Yes, and statistics can tell us what credence to assign to the chance of it happening. I certainly see no reason to assign 50% or similar credence to serendipity in the described case, or indeed any credence high enough to dominate the certain loss of value. Do you?