This paper develops a dynamic model with transaction costs to determine the equilibrium resale pattern in a market for a durable good. The key result is that the probability of resale is nonmonotonic in the age of the good. Trade volume is relatively low in the very beginning and in the middle of a good’s life. This result helps explain observed variations of resale rates across vintages for the U.S. market of used cars.

When a production process requires two extremely complementary inputs, conventional wisdom holds that a firm would always upgrade them simultaneously. We show, however, that if upgrading each input involves a fixed cost, the firm may upgrade them at different dates, “asynchronously.” This insight helps us understand why productivity rises with the age of a plant, why investment in structures is more spiked than equipment investment, and why plants have spare capacity. The bigger point of the paper is that complementarity does not necessarily imply comovement—not even for a single decision maker. (JEL E22, O31, P11)

Tobin's average q has usually been well above 1, but fell below 1 during 1974 – 1984. Our model explains this pattern and reconciles it with unchanging aggregate investment. The stock market value in the numerator of q reflects ownership of physical capital and knowledge, but the denominator measures just physical capital. Therefore, q is usually above 1. Periodic arrivals of important new technologies, such as the microprocessor in the 1970's, suddenly render old knowledge and capital obsolete, causing the stock market to drop. National accounts measures of physical capital miss this rapid obsolescence. Then q appears to drop below 1.