Proof that any person, firm, company, association or corporation has been discriminating between different sections, communities and cities by buying any product, commodity or property of any kind, and paying therefor a higher rate or price in one section, community or city than is paid for the same kind of product, commodity or property by said party in another section, community or city, after making due allowance for the difference, if any, in the grade or quality, and in the actual cost of transportation from the point where the same is purchased to the market where same is sold or intended to be sold, shall be prima facie evidence that the party so discriminating is guilty of unfair discrimination.