原文：

悬牛首于门，而卖马肉于内。这是古代一个寓言故事，意在讽刺那些表里不一、言行不符的人。故事中，商人挂着牛头的幌子，却在店内售卖马肉，这种欺骗行为最终导致了顾客的不满和商誉的丧失。

解析：

这个故事告诉我们，诚实是商业活动的基础。如果一个人或商家只注重表面功夫，而不顾实际品质，最终会失去客户的信任。故事中的商人虽然通过悬挂牛头吸引了顾客，但因为产品质量与宣传不符，失去了顾客的信任，也损害了自己的声誉。这提醒我们在生活中要言行一致，诚信为本。

翻译：

"Hanging a cow's head at the door but selling horse meat inside." This is an ancient fable that satirizes those who are inconsistent in words and deeds. In the story, the merchant hangs a cow's head as a sign but sells horse meat inside the shop. Such deceitful behavior eventually leads to customer dissatisfaction and loss of reputation.

This story tells us that honesty is the foundation of business activities. If a person or merchant only focuses on outward appearances without regard for actual quality, they will ultimately lose customers' trust. The merchant in the story may have attracted customers with the cow's head sign, but because the product quality did not match the promotion, they lost customers' trust and damaged their own reputation. This reminds us in life to be consistent in words and actions, and to value integrity.

总结：

《悬牛首卖马肉》的故事强调了诚信的重要性。无论是做人还是做生意，诚实守信都是赢得他人尊重和信任的关键。希望我们都能从这个故事中汲取教训，在日常生活中坚持诚实守信的原则。