Chapter 4! Selling one property or item requires using those funds to purchase something that has a greater level of sanctity. From a town square to a synagogue to an aron kodesh and onward. Also, a discussion of the sanctity of Jerusalem. And more on this element of kedushat hamakom - sanctity of place. Plus, the story of Rami bar Abba and his desire to use the parts of an old synagogue for the new one he was building. Not such a simple thing to do, according to the rabbis.