(a) It shall be unlawful for any person engaged in commerce, in the course of such commerce, either directly or indirectly, to discriminate in price between different purchasers of commodities of like grade and quality or to discriminate in price between different sections, communities or cities or portions thereof or between different locations in such sections, communities, cities or portions thereof in this state, where the effect of such discriminations may be substantially to lessen competition or tend to create a monopoly in any line of commerce, or to injure, destroy, or prevent competition with any person who either grants or knowingly receives the benefit of such discrimination, or with customers of either of them: provided, that nothing herein contained shall prevent differentials which make only due allowance for differences in the cost of manufacture, sale, or delivery, resulting from the differing methods or quantities in which such commodities are to such purchasers sold or delivered: and provided further, that nothing herein contained shall prevent persons engaged in selling goods, wares, or merchandise in commerce from selecting their own customers in bona fide transactions and not in restraint of trade: and provided further, that nothing herein contained shall prevent price changes from time to time where in response to changing conditions affecting the market for or the marketability of the goods concerned, such as but not limited to actual or imminent deterioration of perishable goods, obsolescence of seasonable goods, distress sales under court process, or sales in good faith in discontinuance of business in the goods concerned.