With the ever-growing popularity of online shopping, platform environments providing access to products by multiple sellers increasingly attract users. To reduce information asymmetry and enhance user trust, platform actors provide signals such as star reviews to demonstrate their trustworthiness. This work investigates the influence of trust signals from different sources (on the platform itself vs. on external third-party review sites) and for different targets (platform provider vs. seller) on users’ trust formation in multi-sided e-commerce platforms. We conduct a choice-based conjoint analysis based on data from 81 participants. Our results show that users weigh external signals stronger than internal ones when building trust. Also, trust signals for sellers have a higher impact on users’ trust than platform provider signals. Signal discrepancies between internal and external reviews are especially harmful to the platform provider. These insights extend prior knowledge on trust formation and its impacting factors on e-commerce platforms.