Which of the following is NOT an assumption of imperfect competition?

In the context of imperfect competition, one of the key assumptions is that there are many sellers within the market. This characteristic distinguishes imperfect competition from perfect competition and monopoly. In an imperfectly competitive market, such as monopolistic competition or oligopoly, while there may be many sellers, they still have some degree of control over the prices due to product differentiation.

High product differentiation is another hallmark of imperfect competition. It implies that the products offered by different sellers are not identical, which allows sellers to influence their pricing based on brand loyalty and perceived quality differences among consumers.

The assumption of freedom of entry and exit is also essential in defining market structures. In imperfect competition, firms can enter and exit the market relatively freely, which differentiates it from a monopoly, where barriers to entry are usually high.

However, the presence of only one major seller in the market describes a monopolistic market structure, not imperfect competition. In monopolistic scenarios, a single firm dominates the market, thereby eliminating competition entirely. This is why having only one major seller is not consistent with the assumptions of imperfect competition.