What is one of the conditions necessary for the Robertson-Patman Act to be applicable?

The Robertson-Patman Act is a U.S. federal law aimed at preventing anticompetitive practices by ensuring that price discrimination does not harm competition. One of the essential conditions for the applicability of the Act is that there must be a price difference charged by the same seller for the same product sold to different purchasers. This means that if a seller offers the same product at different prices to different buyers, it potentially violates the Act unless certain exceptions apply, such as those based on volume discounts or cost differences in serving different purchasers.

The Act is fundamentally about protecting competition and ensuring fairness in pricing practices, which is why the price difference by the same seller is a crucial element. Without this condition being met, the applicability of the Act cannot come into play, as it is specifically targeted at scenarios where one seller's pricing leads to an unfair advantage over another competitor by offering disparate pricing for the same good.

In contrast, the other options do not capture the essence of the Act's focus. Sales conducted internationally are subject to different regulations, and involving multiple product grades or sales of different commodities does not inherently relate to the core principle of price discrimination that the Robertson-Patman Act is designed to address.