Platforms can steer demand by selectively promoting complements in platform markets. But how does selective promotion affect the overall demand when the products are non-fungible, such as on auction platforms, and does this effect differ for adjacent products and a competing platform? By studying a car auction platform with unique cars, we find that promotional car reviews on YouTube positively affect the prices and bid volumes of reviewed cars in the same category. However, the latter effect is largely due to short-term attention spillovers, while the sentiment of the reviews can have unexpected consequences for the bid prices. Moreover, the effect on the competing platform is rather limited and probably only occurs when users first visit the focal platform and then switch to the competing platform.
