The sentence is discussing a commercial success formula based on swindling or exploiting others. "Buy cheap and sell dear" is a well-known economic phrase that refers to the practice of buying goods at a low price and selling them at a higher price, which is a common commercial strategy.
Option (A), sell what you cannot buy back, doesn’t fit because it implies an exchange where something is sold but cannot be reclaimed, which doesn’t align with the concept of profit-making.
Option (B), buy what you will sell to another at a higher price, is somewhat close but doesn't capture the essence of buying cheap and selling at a high profit.
Option (C), sell what you can, do not buy from a competitor, is not a good fit because it introduces a concept of competition rather than focusing on the strategy of buying at a low price and selling at a high price.
Thus, option (D) is the most appropriate choice.