What clues are provided about Mr. Shiftlet's selfish intentions even though his words and actions simultaneously contradict those clues?
The first clue is his name. "Shiftlet" would mean "allowing one to shift" (change, not be reliable), and is close to "shiftless," which implies a kind of low greed.
The old woman's reaction is another clue; she sees him as a tramp.
O'Connor also provides clues in her description of Mr. Shiftlet; he has a "jutting steel‑trap jaw," and a steel trap would betray people, clamp down on things and not let go, holding them tight. Later, his gaze is described as "sharp," which is, among other things, not friendly.