In Money:Tech Prediction Markets, we do not force a buyer to be matched with a seller, and vice versa. From a trader's perspective, Money:Tech Prediction Markets is always the buyer and seller of shares and there is no limit to the number of shares in play. Money:Tech Prediction Markets also sets the stock price according to demand or lack there of. If a trader buys shares, there is demand for the stock and its price goes up. If a trader sells, there is a lack of demand and the price goes down.
The principles behind our algorithms originate from research by Professor Robin Hanson at George Mason University. Here is some background about Hanson's automated market maker: