In the book Uncle Tom's Cabin the dealers try to get the slaves to act cheerful. Why do the slaves comply with their directions?

THey really have no choice. They will be punished otherwise.

They act accordingly in order to get a good master if possible.

"Briskness, alertness, and cheerfulness of appearance, especially before observers, are constantly enforced upon them, both by the hope of thereby getting a good master, and the fear of all that the driver may bring upon them if they prove unsalable."