Are there any surefire methods to thoroughly clean preloved items? This includes clothes, shoes, toys and books

For clothes, I suggest you either soak them in baby-friendly bleach like Oxyclean or give them a hot wash in the laundry. For shoes, brush with bleach, too. As for toys, I suggest you just use natural ingredients like baking soda and vinegar as they have anti-bacterial properties. For books, I just wipe them with alcohol