Efficient-market theory assumed investors always had the means to act: If they knew that a share of IBM was worth $90 rather than the prevailing price of $100, they would sell it short until the weight of their trading moved the price down by $10. This assuming away of institutional frictions involved a number of heroic leaps. You had to presume that the knowledgeable speculators could find enough IBM stocks to borrow in order to be able to sell them short. And you had to gloss over the fact that, in real life, the “knowledge” that IBM was worth $90 would be less than certain. Speculation
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