Gemara

Reuven took a butchers' knife for a security from Shimon.

Abaye: Since it is used for food, you must return it. You can claim your loan in Beis Din (if you have witnesses, or if Shimon will admit.)

Rava: Since witnesses did not see Reuven take it, he need not return it until Shimon pays.

Question: Abaye should agree to Rava like the following case!

Some goats ate peeled barley in Neharde'a. The owner of the barley seized the goats, and was claiming a large loss.

(Shmuel's father): He can claim up to the goats' value.

Answer: Goats are not normally lent or rented, so one who seized is believed, Migo (since) he could have said that he bought them. A knife is normally lent or rented. One would not be believed to say that he bought it.

(Rav Huna bar Avin): If Ploni holds an item normally lent or rented, and claims that he bought it (if we know that it was David's), he is not believed.

Question: Does Rava disagree?! Rava took shearing scissors and a Sefer of Agadata from orphans, because these are normally lent and rented!

Answer: A butchers' knife can get blunted, so people do not normally lend or rent it.

Kesuvos 15b (Mishnah): If Levi says to David 'this field belonged to your father and I bought it from him', Levi is believed. The one who forbids is believed to permit (without his admission, David had no claim);

If witnesses testify that the field belonged to David's father, and Levi says 'I bought it from him', he is not believed.

84b: A shepherd was watching cattle of orphans. A creditor seized a cow. He said that he seized it in the father's lifetime; the shepherd disagreed.

Rav Nachman: If there are no witnesses of seizure, since he could claim that he bought it, he is believed to say that he seized in the father's lifetime.

Question: Reish Lakish taught that custody of an animal does not prove ownership! (Perhaps he found it wandering outside.)

Answer: He did not refer to animals being watched by a shepherd.

Rishonim

Rif: If Ploni deposited or lent to Levi with witnesses, even if the deposit is something not normally lent or rented, Levi is not believed to say that he later bought it. We needed R. Chanina (our text - Rav Huna) bar Avin's law only for when it was deposited not in front of witnesses.

Rambam (Hilchos To'en 8:3): (If Ploni claims that Levi has his item,) Levi is believed to say that he bought it or took it for a security if it is something not normally lent or rented, such as clothing, Peros, house utensils and merchandise.

Rambam (10): If a Kli could be rented out, but it is prone to depreciate more than the rental, and people are particular not to lend it, it is Muchzak not to be lent or rented, e.g. a knife for Shechitah. Therefore, even if witnesses testified that he lent or rented it, its Chazakah is intact. It is like all Kelim. A proof is that Rava took shearing scissors and a Sefer of Agadata from orphans, which are normally lent and rented. He would not have done so unless witnesses clarified that they are normally lent or rented. Other scissors and Seforim are not in this category, even though one could lent or rented them. This is a great general rule in monetary laws. It is reasonable; one may rely on it.

Magid Mishneh: The Rambam explains that Abaye and Rava argue in any case when witnesses said that the owner lends it. If not, Abaye would not consider it something often lent or rented! Rava holds that we do not alter its status because the owner acted differently. He caused his own loss! There is no proof from the Gemara that Rava had a proof through witnesses. Perhaps the Rambam had a different text.

Lechem Mishneh: The Rambam learned from the words 'Rava took scissors... from orphans, in things normally lent.' It did not say because these are normally lent. The wording used teaches that witnesses can make something considered a Kli normally lent or rented.

Bach (CM 72:13): How can the Rambam say that Chazakah overrides witnesses?! Perhaps he means that even though witnesses testified that he lent or rented it, presumably it was returned, for it is not normally lent. Therefore, the borrower can say that after returning it, he bought it. All the Poskim are like the Rif and Tur.

Rebuttal (Shach 72:83): The Rambam means that witnesses testified that the owner once lent it to someone else. The Magid Mishneh connotes like this. All agree to this law.

Rosh (Bava Metzia 9:48,49): The Halachah follows Rava. If witnesses know that Ploni owned something often lent or rented and it was found in Levi's Reshus, or if Ploni deposited something not normally lent or rented with Levi in front of witnesses, Levi is not believed to say that he bought it or took it for a security if witnesses saw that he has it.

Hagahos Ashri: Ploni takes it without an oath. This is only if he often lends, Levi often borrows, and it depends on the level of love or ill will between them, like the judges assess.

Rosh (ibid.): If witnesses did not see that he has it, he is believed, Migo he could say that he returned it. Similar, if something is not normally lent or rented, and it was not deposited in front of witnesses, even if witnesses saw that Levi has it, he is believed to say that he bought it or took it for a security.

Hagahos Ashri: If Levi died or sold it, we claim for the heir or buyer that Levi bought it, for Levi would have been believed to say so. Even though Levi would have had to swear Heses, the heir or buyer need not. R. Tam says that if it was well-known that Levi has it, he has no Migo to say that he returned it. The Ri was astounded; this does not Mevatel the Migo. However, if witnesses saw it, Reuven can take his inheritance from Shimon. Even though Reuven does not know if it was borrowed or bought, he can Mevatel the Chazakah of the one who took possession with a Vadai claim.

Hagahos Maimoniyos (Mishpatim, Teshuvah 32): We say in Kesuvos that if animals were not watched by a shepherd, one would not be believed to say that he seized in the father's lifetime, since there is no Chazakah on them (for anyone can take them.)

Shach (CM 72:88): Another proof for the Ri is from Kesuvos 15b. If witnesses testify that the field belonged to David's father, and Levi says 'I bought it from him', he is not believed. There is no reason to distinguish land from Metaltelim that are normally lent or rented. All the more so, if Reuven has witnesses that his father lent to him, even though he does not know whether or not he bought it later, if it was now seen with Shimon, Reuven takes it from him even if it is something not normally lent or rented.

Poskim

Shulchan Aruch (CM 72:18): In any case when Reuven (the lender) could not claim 'I bought it (the security)' or 'it was never yours' or 'I returned it to you', Shimon (the borrower) swears that he borrowed only a Shekel, pays it, and takes his security. Regarding something not normally lent or rented, if there are no witnesses how it came to Reuven, he is believed to say that he bought it.

SMA (56): This is even if witnesses saw Shimon give it to Reuven, but they do not know if it was a deposit, security, sale or gift.

Shulchan Aruch (ibid): The same applies if witnesses saw Reuven borrow it, but did not see it now with him. He can say that he returned it, or 'nothing ever happened' if witnesses did not see Shimon give it to him.

Shach (85): Since he could say that he returned it or 'nothing ever happened', he is believed through a Migo to say that it is a security for a loan of such and such amount.

Shach (86): The Mordechai (Bava Basra 542) and Terumas ha'Deshen (335) say that the Rif and Rambam hold that he is not believed to say that he returned to when there are witnesses that he borrowed. Other Poskim, the Beis Yosef and Magid Mishneh (Hilchos She'elah 6:4) say that the RIf and Rambam agree (that he is believed). Even though the RIf and Rambam did not mention that witnesses saw it now, I say that they require it (to remove the Migo), for the Migo 'I returned it' is a proper Migo. Bedek ha'Bayis (Siman 133:4) explains the Rif and Rambam like this. This is the Halachah.

Shulchan Aruch (90:13): One holding something not normally lent or rented is believed (to say that it is his) only if witnesses did not know that he initially borrowed or rented it. If they knew, he is not believed.