If marginal net utility is positive, the consumer must be buying too small a quantity to maximize total net utility. Why?

Because marginal utility exceeds price, the consumer can increase total net utility further by buying (at least) one more unit of the product. In other words, since marginal net utility (which is marginal utility minus price) tells us how much the purchase of an additional unit raises or lowers total net utility, a positive marginal net utility means that total net utility is still going uphill.