Why did doctors examine the slaves before they were purchased in africa?

The doctors used to  carefully examine the slaves at the time of purchase in Africa since they knew that the healthiest ones had the best chance of surviving the transport (trans-Atlantic crossing) and fetching a high price in America.

Slave holders and doctors  focused on "soundness", a concept originally used to asses livestock, to gauge an enslaved person's overall state of health and his or her worth in the market place. Soundness was a means by which slaveholders and the doctors they employed defined the capacity to work, reproduce and submit to authority.