1) I did once hear that the Meiri writes somewhere that most of the Halachos in Maseches Bava Basra are based on sevarah. I cannot find at the moment where the Meiri writes this. However I understand the idea behind this to be that if we look at Bava Kama we see, for instance, that it opens with the damages of "ox", "pit", "fire" etc. which are stated explicitly in Parshas Mishpatim. Then in Bava Metziah, for instance, we have the Mitzvah of returning lost property which is also learnt from explicit verses. Bava Basra is different because we do not have so many Halachos which are learnt from verses.

2) However, after saying that, it should be pointed out that the third perek does open with Rabbi Yochanan saying that chazoka of 3 years is derived from the ox which is considered "Muad" if it gores 3 times. The Gemara proceeds to cite further sources for 3 years. We observe that the Gemara is trying hard to find the source for chazoka.

3) The Mishnah Kidushin 26a tells us that land is acquired through money, a deed of sale, or chazokah. [Rashi writes that one locked the gate of the field to show that one is acting like owners do. This does not refer to a 3-year chazoka]. The Gemara derives the above Halachos from verses.

4) The Gemara Bava Basra 37a cites the opinion of Rabbi Akiva that when someone makes a sale he does so generously ("Mocher b'Ayin Yafe Mocher). This must be an estimation of the average person's intentions when he makes a sale, so it is a sevarah about public opinion.

Shlomo Zalman, to answer your question properly, we will have to learn the whole perek.

Chodesh Tov

Dovid Bloom

1) I have still not found that Meiri, but I have found that the Tiferes Yisrael writes something similar.

It must be very significant that the very last Mishnah in Maseches Bava Basra 175b tells us: "Rabbi Yishmael said 'if someone wishes to be wise, he should occupy himself with the Dinim of finance, because there is no similar area of Torah, which is like an overflowing spring' ".

2) Tiferes Yisrael, at the end of the Masechta #85 writes that there is nowhere else that the Torah granted so much permission to human intelligence to fly around and research penetratingly, and to decide according to what seems logical, as we find in financial Halacha. The Torah opened up here an "unlimited inheritance". The Dayan is entiltled to weigh up and judge according to his intelligence. Tiferes Yisrael writes that the majority of the vast field of laws of plaintiff and defendant, are included in a few words of the Torah in one verse (Vayikra 19:15) "Judge your fellow righteously".

3) Tiferes Yisrael writes that the Rabbis of the Mishnah and Gemara, thousands of the great minds of the nation, gathered together and laid down principles with which to make rulings, but the Judge is still left with space to use his own logic, sometimes making a distinction as delicate as a hairsbreadth. This is why studying monetary Halacha sharpens ones intellect. In areas of the Torah dealing with what is forbidden and what is permitted, if one is unsure what the Halacha should be, one can be stringent in case of doubt. In financial matters one cannot do so, because if one is stringent on one of the parties, this means one is lenient on his opponent, which is unfair. Therefore one has to put all one's efforts into achieving the totally true ruling.

4) We learn from Tiferes Yisrael that the source for thousands of Halachos is from 3 words of the Torah "Betzedek Tishpot Amitecha" (Vayikra 19:15). Tiferes Yisrael does not write that this necessarliy refers more to the 3rd chapter of Bava Basra than to other chapters, but it is meaningful that the last Mishna in Bava Basra teaches us this special nature of monetary law, because Bava Basra is so full of Halachas based on sevarah, and it is well known that the 3rd chapter of Bava Basra is special in its content of sugyos which are based on fine logic.

Good Shabbos

Dovid Bloom