Flannery O'Connor's short story "Good Country People" was first published in 1955. O'Connor herself grew up in a small town in Georgia and many of her stories have a small town, rural setting, as is the case in "Good Country People."
O'Connor's father died of lupus when she was young and so her mother ended up running the family's dairy farm. O'Connor came back to this farm in the 1950s after she herself was diagnosed with lupus.
Thus, the rural farm in the southern part of the United States, as described in "Good Country People," is one with which O'Connor must have been extremely familiar.
Likewise, unusual visitors that might have stopped by the farm must also have been familiar to O'Connor. Wandering salesmen like Manley Pointer, who tricks Joy-Hulga, were probably based on real-life figures that O'Connor had seen come and go from her own family's farm.
The isolated setting of the farm supports the theme of innocence versus intelligence. Joy-Hulga is educated from books, but she has no experience of the world. Manley Pointer is a seller of books, but he has no knowledge of information that comes from books. He does, however, have worldly knowledge and he uses this knowledge to seduce Joy-Hulga and trick her into taking off her leg, which he then steals. Joy-Hulga had imagined that she would easily seduce Manley Pointer, but her "book-learning" was no match for the worldly knowledge possessed by Manley Pointer.