In this case, we see a figure of speech, called synecdoche, where a part of the body (feet) represents the whole.1 Peter is pointing out that the men who buried Sapphira’s husband have just returned from doing so and that they will now carry her dead body out for burial as well. The same fate which befell her husband will also come to her—death and immediate burial.
9 But Peter said to her, “How is it that you have agreed together to test the Spirit of the Lord? Behold, the feet of those who have buried your husband are at the door, and they will carry you out.”