While observational studies have long suggested a positive correlation between social relationships and online transactions, surprisingly little research demonstrates a causal link. Effects identified in observational data generally conflate the Information effect, which bears the counterfactual causal interpretation, with the Homophily/environment effect. Against this background, this study conducted a pioneering a randomized field experiment design to isolate the Information effect of friendship disclosure from confounding homophily factors. We exploit a rare opportunity to conduct a field experiment on a large Chinese online second-hand platform, in which we manipulate buyer and seller’s awareness of their preexisting friendship ties. We provide the first empirical evidence that the effect of revealing friendship information between transaction parties turns out to be insignificant. We demonstrate that reliance on observational estimates of the “total effect” of friendship significantly overstates the benefits of providing friendship information in online marketplaces. Our findings contribute to a better understanding of social commerce and highlight the potential fallacy of relying on observational data in business studies.