“in the hearing.” The Hebrew text is idiomatic: “in the ears of the sons of Heth.” The elders of the Hittites were there to witness the negotiations between Abraham and them for a piece of the land owned by whichever one of them had the land that Abraham wanted. The witnessing of this event is repeated using “eyes” instead of “ears” in Genesis 23:18.
“all those who come to the gate of his city.” This idiomatic expression means more than simply, “those who go in and out of the city,” it refers to the “elders at the gate,” the important people and decision-makers in the city. Obviously, the whole city was not present for the negotiations between Abraham and Ephron, but the important people of the city were all there. The gate of the city was not just a door, it was a “gate area,” usually with an inner and outer gate, and places for sitting inside the gate area. So even at this early date, around 2,000 BC, Hebron was a walled city, and part of the city wall from this time has been discovered by archaeologists. This same phrase is used in Genesis 23:18.
[For more information on the elders at the gate, and that a person could seek wise advice there, see commentary on Proverbs 1:21.]