Every corporation, association or person, now or hereafter claiming or exercising the right to carry or transport natural gas by pipe line or lines, for hire, compensation, or otherwise, within the limits of this state, as owner, lessee, licensee, or by virtue of any other right or claim, or now engaged or hereafter engaging in the business of purchasing and selling natural gas shall be a common purchaser thereof, and shall purchase all the natural gas in the vicinity of, or which may be reasonably reached by its pipe lines, or gathering branches, without discrimination in favor of 1 producer or 1 person as against another, and shall fully perform all the duties of a common purchaser; but if it shall be unable to perform the same, or be legally excused from purchasing and transporting all the natural gas produced or offered, then it shall purchase and transport natural gas from each person or producer ratably, in proportion to the average production, and such common purchasers are hereby expressly prohibited from discriminating in price or amount for like grades of natural gas or facilities as between producers or persons; and in the event it is likewise a producer, it is hereby prohibited from discrimination in favor of its own production, or production in which it may be interested directly or indirectly, in whole or in part, and its own production shall be treated as that of any other person or producer.