Is it credible for fitness instructors to buy muscle powder online?

Not credible.

Muscle building powder is not suitable for everyone. Muscle tissue can be increased by its own ability, not by hormones. Harmful and useless.

The three best intake times of muscle-building powder are: after getting up early, after training, and before going to bed (as for this meal before going to bed, you can eat it or not, because muscle-building powder is not protein powder, which is high in calories and fresh water, and it is easy to get fat before going to bed), and eating a few egg whites 1 hour before going to bed is enough.