Why would a slave whose life on a plantation was very bad fear being sold to a slave-trader?

There are a couple of reasons for which a slave might express such a fear.
Firstly, there was the possibility of being forced away from one's family, never to see them again. Though slaves were not allowed to marry legally, they still partook in marriage ceremonies, referred to as "jumping the broom," and retained as much love and dedication to their loved ones as those who were free. Being sold meant being forced to live on a plantation or a small farm far away from one's spouse or children, sometimes even in another state.
Being obedient did not always help to avoid this. Because black men and women were regarded as property, they were usually assessed according to monetary value. Slave owners who required revenue might have been inclined to sell off a man or woman who was an exceptionally good field hand. An exceptional house slave (e.g., a cook, a wet nurse) might have gotten the same treatment. A slave's stability was always uncertain.
A second reason for fearing sale was that, no matter how bad one's master was, there was always the possibility of being sold to someone much worse. Even when a slave was subjected to a cruel master, living with him or her for long enough allowed the slave to anticipate the worst moods and to adapt to them; being sold to someone else required new modes of adaptation. While it was unlikely that a slave master would murder a slave (though, as Douglass demonstrates in his narrative, not at all impossible), due to fearing loss of revenue, one could be subjected to forms of cruelty that would make death seem preferable.