Social platforms are currently encountering a set of burning issues: low ad conversion rates, cross-channel free-riding phenomena, lack of monetary incentives to retain premium content creators, etc. Direct purchase behaviors between social platform users (e.g., making a direct purchase through a seller's promotional post) can largely resolve these problems. Therefore, it is imperative to study the factors that influence users' direct purchase behavior. This paper focuses on risk- and trust-related factors, proposing a theoretical model that was tested on two samples of Chinese users of WeChat. The authors concluded that users tend to evaluate the shopping risk associated with the social platform first, then go through a process of building trust in the platform before making purchases. Further, this trust can generate a halo effect on seller risk. Finally, trust and seller risk directly impact on users' purchase intention to buy from the seller on the platform.