a firm under perfect competition is …

In perfect competition, there are two main reasons why a firm cannot get away with setting its prices above the market price.  First, there is no difference between its product and that of every other firm in the market. Therefore, no one will pay extra for a firm’s product. If a firm tries to charge a higher price, buyers will go with other sellers, while at a lower price, the firm will not be able to cope with demand due to a large number of buyers. 

Second, if a firm were to succeed in setting a higher price, more firms would enter the market, attracted by the higher profits that were available.  This would increase supply and drive down the price of the firm’s product. These two factors make it impossible for firms to set their prices above the market price.  This makes them into price takers.