Answer:
They could be legally killed or sold.
Step-by-step explanation:
Free African-Americans required a certificate to verify their freedom because there were many people willing to sell them. Seeing an African American walking around town wasn't common unless they were collecting water or running errand for their owners. So, naturally people grew suspicious.
Also, many runaway slaves could say that they were free. And, there would be no way to verify that if there were no certificates .
Slavery was truly a terrible time.