Gemara

Moshe claimed that he bought David's house, and lived there for three years. David said 'I did not sell it. I was doing commerce abroad.' (This is why I did not protest.)

Moshe: I have witnesses that you returned for 30 days every year!

David: All 30 days, I was busy in commerce.

(Rava): It is normal that one is busy in commerce for 30 days.

Rishonim

Rif and Rosh (15b and 3:6): One can say that all 30 days that he was here, he was busy in commerce and did not know that someone else was in his house.

Rif: Rav Hai Gaon says that this episode was at a time of Cherem. At other times, the Muchzak need not be in the same country, for a protest not in front of the Machazik is valid. Presumably, he is correct.

Nimukei Yosef (DH Lamed): The Muchzak need not be in a place where where he could protest for all three years. It depends only on the end of the three years. Even if he was in a place where he could protest, and went (at the end) to a place where he cannot, the Chazakah is Batel, for we can say that he intended to protest when he returns, but did not have time.

Rosh: Some learn from here that if the Muchzak says that he was not in the country for the three years of Chazakah, he is believed, and the Machazik must prove that he was. I say that it is no proof. Perhaps it was known that he was abroad! It seems that he need not bring a proof. Since the Machazik made a Chazakah, word spreads even to other countries. The Muchzak must prove that he was in a place where his protest would not have been heard, and therefore he did not protest.

Drishah (CM 143:2 DH v'Chol): The Rosh says that since there is no proof that the Muchzak is believed, it is more reasonable to say that he is not believed. Even though we discuss the Muchzak hearing about the Chazakah, he says that the Muchzak must prove that he was in a place where his protest would not have been heard, for we assume that he surely heard.

SMA (143:5): The Rosh teaches that not only he is not believed to say that he did not hear at all; rather, even if he says that he heard close to the end of three years and there was no time for his protest to be heard, he is not believed.

Rambam (Hilchos To'en 11:3): If Reuven brought witnesses that Shimon came here for 30 days or less every year, we tell Shimon 'why didn't you protest when you came? You lost your rights.' If he claims 'I was distracted with the market and I did not know that he is in my Chatzer', we accept this claim. (Often) one is distracted with business for 30 days. If he tarried more than 30 days and didn't protest, he lost his rights. It seems to me that this law is only in villages where people are distracted with their markets.

Kesef Mishneh: There is more distraction in the city than in the villages! Perhaps he refers to great fairs in which multitudes gather in the villages, for there is more room there than in the city.

SMA (CM 143:3): I say that in the city, there is only one market day each week, so there is less distraction.

Bach (DH u'Mah she'Chosav): In the city, there is a wall and many rooms (in houses), so it is easy to guard one's money. In the village, people must dwell in tents and they are distracted to guard their money from marauding bandits.

Poskim

Shulchan Aruch (CM 143:1): A Chazakah for three years is valid even if it was not in front of the Muchzak, even if he was in a different country, if caravans are frequent between them. If there was war or the roads were ruined, it is not a Chazakah. Even if witnesses say that the Muchzak came to the market here and tarried 30 days at the end of the three years, and if he does not protest now he will not be able to protest later, he can say that all 30 days he was distracted with business and did not know that someone is Machazik in his house. This is when the Muchzak had another house to stay in. o Some say that this law is only in villages where people are distracted with their markets.

Beis Yosef (DH v'Afilu): The Rashbam says that Reuven can say that all 30 days he was busy in commerce and did not notice that someone was in his house. Others explain that he was busy and had no time to protest. The first opinion is primary; the Rif holds like it.

Ketzos ha'Choshen (1): The Tur says that it is easy to protest in front of two, so he cannot claim that he was too busy to protest. Tosfos and the Rosh say that Chazakah is heard even in a different country, but Macha'ah is not. If so, why is distraction with the market an excuse? He already heard beforehand! He should have protested when he returned to his city, for it is easy! We must say that he was so distracted with the market that he forgot about the Chazakah.

Rema: Some say that we learn from here that it depends on the end of the three years. If at the beginning of the three years the Muchzak was in a place where he could not protest, and at the end of the three years he was in a place where he could protest, it is a Chazakah. If at the beginning he was in a place where he could protest, and at the end he was in a place where he could not protest, it is not a Chazakah. He did not protest at the beginning because he planned to protest at the end and return, but he did not have the opportunity.

SMA (4): This opinion learns from the need to say that he was distracted with the market. If not, the Chazakah would be valid, even though he was there only at the end of three years. We also infer oppositely, that if he was there only at the end of three years, he has an excuse to say that he planned to protest later.

Beis Yosef (DH u'Mah she'Chosav Rabeinu v'Hayu): The Nimukei Yosef says so. This is only like those who explain that because he was busy, he did not have time to protest.

Rebuttal #1 (Darchei Moshe 1): The Nimukei Yosef says that perhaps he intended to come later (and protest). This applies according to both opinions!

Rebuttal #2 (Bach DH Kosav): The Nimukei Yosef said that he did not know about the Chazakah!

Hagahos Drishah (1): The Nimukei Yosef wrote only the Perush that he did not know about the Chazakah!

Hagahos Tur ha'Shalem (7): Keneses ha'Gedolah says that the Beis Yosef had a different text of the Nimukei Yosef.

Defense (of Beis Yosef - Shach 1): The Beis Yosef means that if Reuven knew that Shimon wil make a Chazakah and Reuven will want to claim that he was distracted and did not know, this is not a claim, and Shimon has a Chazakah.

Ketzos ha'Choshen (2): If it depends only on the end, why did the Gemara say that he was away the rest of the time? Even if he was here, there is no Chazakah, for the last 30 days he was distracted! The Nimukei Yosef is like the opinion in the Rashbam that the Chazakah is Batel when he intended to protest at the end, but did not have time. This is like one who intended to pray later, and was unable due to Ones. If there was no Ones, just he did not have free time, what was his excuse for not protesting? The Beis Yosef says that the Nimukei Yosef is like the latter opinion. I explain that the Rashbam explains that he did not protest because he forgot. This is total Ones. This is even if he was here the entire three years! Therefore, we must say that the Nimukei Yosef holds like the other Meforshim.

Nesivos ha'Mishpat (Bi'urim 2): The Nimukei Yosef says that even if he could have protested when he came the previous years, since he was distracted at the end of the three years, it is not a Chazakah. The Gemara needed to say the rest of the time he was far away (from where Macha'ah would not be heard), for otherwise he should have realized that he will be distracted with the market, and he should have protested earlier. This is if the problem is inability to protest. According to the opinion that disqualifies the Chazakah because the Muchzak did not know, since he knew beforehand, he was obligated to protest lest he be distracted at the end! This is the Beis Yosef's proof.