Under a Buttonwood Tree - the exchange, the order book and the spread

That board’s auctions each have one seller and a crowd of buyers, and close when the thing is sold. This lesson runs an auction that never closes, with buyers and sellers both queuing at once, which is what a stock exchange is.

Before you start. You need two ideas. From Owning a Slice of the Ship - shares, bonds, and who gets paid first: a share is a slice of a company that you can sell to someone else, which only helps if there is somewhere to find that someone. From Auctions & Mechanism Design: an auction is a rule for finding out who wants a thing most and what they will pay; you do not need Vickrey’s second-price trick here, only the idea that the rules of a sale decide who wins it.