It's user-dependent. Stripe provides users with some protections (e.g. against card-testing), and then users will verify their platform's sellers in a way that makes sense for them. Lyft and Sidecar collect drivers' licenses from drivers, and only pays them out in proportion to the amount they drive. Homejoy trains cleaners, and each user rates the cleaner. And so on. Fraud is heterogenous by nature.