For items with strict industry standards, try to find unfamiliar sources for non-standard products, and for familiar sources, work with people you've collaborated with before. For example, when you buy an Apple iPhone, don't just look for a familiar seller to save a little money, because every iPhone leaving the factory is identical, and the manufacturer provides warranty even if it breaks. As long as the channel you choose can guarantee genuine products, that's fine. For non-standard items, from small things like haircuts and private tailors to large-scale software customization and development, you should try to work with trusted acquaintances you've collaborated with before!