Miss Ophelia's purchase of Topsy most clearly underlines that there is no such thing as a "good" slaveholder.
When Miss Ophelia receives Topsy, she becomes aware that no matter what people think, or believe or what ideas or even values they support, as long as it is beneficial for them they may do the contrary.
Miss Ophelia in Uncle Tom's Cabin is a pious, hard-working, abolitionist from the North. She's also unmarried, and beyond marrying age.