Chagigah 26A

או שניהן יוצאין ⁠— כלחוץ.

or both are leaving it, it is considered like outside the perimeter and the ḥaver may not acquire vessels from him. The reason is that if they are both entering the perimeter they can easily wait until they are inside and then conduct the transaction, and if they are both leaving they should have completed the deal beforehand, and the ḥaver may not make up for this lapse by doing so now.

אמר אביי, אף אנן נמי תנינא: הקדר שמכר את הקדירות ונכנס לפנים מן המודיעים. טעמא דלפנים מן המודיעים, הא מודיעים גופה ⁠— לא מהימן. אימא סיפא: יצא ⁠— אינו נאמן. הא מודיעים גופה ⁠— נאמן. אלא לאו שמע מינה: כאן בקדר יוצא וחבר נכנס, כאן בששניהן יוצאין או שניהן נכנסין. שמע מינה.

Abaye said: We, too, learn this in the Mishnah. For it is taught there: A potter who was selling pots and entered within the Modi’im area is deemed credible, which indicates that the only reason he is deemed credible is that he is inside the Modi’im area, thus implying that in Modi’im itself he is not deemed credible. But now say the latter clause of the Mishnah: If he left he is not deemed credible, thus implying that in Modi’im itself he is deemed credible, which contradicts the previous inference. Rather, must one not conclude from the Mishnah the following distinction: Here, in the latter clause, it is referring to a potter who is leaving and a ḥaver who is entering, in which case he is deemed credible; and there, in the first clause, it is referring to a situation where they are both leaving or both entering, in which case he is not deemed credible. Consequently, both inferences from the Mishnah are upheld. The Talmud concludes: Indeed, learn from here that this is the case.

תנא: נאמנין בכלי חרס הדקין לקודש. אמר ריש לקיש: והוא שניטלין בידו אחת. ורבי יוחנן אמר: אפילו שאין ניטלין בידו אחת.

§ A tanna taught in the Tosefta (3:33): All people, including amei ha’aretz, are deemed credible with regard to purity from Modi’im and inward only with regard to small earthenware vessels, and they may be used for sacrificial food. Since these small vessels were needed by all, the rabbis deemed the amei ha’aretz credible concerning them. The amora’im discussed the meaning of the term small vessels. Reish Lakish said: It is speaking of those vessels that can be picked up in one hand, but no larger. And R' Yoḥanan said: Even if they cannot be picked up in one hand, they can still be called small vessels.