1911 Encyclopædia Britannica/Buying In
BUYING IN, on the English stock exchange, a transaction by which, if a member has sold securities which he fails to deliver on settling day, or any of the succeeding ten days following the settlement, the buyer may give instructions to a stock exchange official to "buy in" the stock required. The official announces the quantity of stock, and the purpose for which he requires it, and whoever sells the stock must be prepared to deliver it immediately. The original seller has to pay the difference between the two prices, if the latter is higher than the original contract price. A similar practice, termed "selling out," prevails when a purchaser fails to take up his securities.