Related על דמאי 2:3

One takes upon himself four obligations that are placed on him to be a "chaver" (see Dem. 2:3): (1) he does not sell terumah or ma'aser to an am ha'aretz [a Jew who is not scrupulous in his observance, including the laws of purity and tithing], (2) he does not engage in cleanliness [?] at the home of an am ha'aretz, and (3) he [only] eats unconsecrated meat in a [state of] ritual cleanliness. [Note: per the GR"A, we add (4) he does not sell [Demai] to an am ha'aretz.]