My father had a rather clever way of explaining the bid-ask spread: “Every time you have an exchange of goods and services between two parties, you have an agr

My father had a rather clever way of explaining the bid-ask spread: “Every time you have an exchange of goods and services between two parties, you have an agreement on price and a disagreement on value.” i.e. Given an exchange between a seller and a buyer selling a widget for $100, the seller by definition values the $100 more than the widget while the buyer values the widget more than they value the $100.