Gemara

(Mishnah): A craftsman does not get a Chazakah...

(Rabah): This is only if witnesses saw the owner (Reuven) give Shimon (the craftsman) the Keli. If not, Migo (Since) that Shimon would be believed if he denied ever receiving it, he is believed to say that he bought it.

Question (Rav Nachman bar Yitzchak): The Mishnah says that a craftsman does not get a Chazakah. This implies that others get a Chazakah! If witnesses saw it deposited, the Shomer would not get a Chazakah! Rather, there are no witnesses, and even so a craftsman does not get a Chazakah!

This refutes Rabah.

Bava Metzia 116a: 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.

Rishonim

Rif (Bava Metzia 70a): The Halachah follows Rava. We learn from our Sugya that if Ploni deposited or lent to Levi, even without witnesses, something often lent or rented, and witnesses know that it was Ploni's, Levi is not believed to say that he bought it or took it for a security. Even if Levi died and his children claimed this, they are not believed, just like he was not believed. Rava took scissors and a Sefer from orphans, for they are normally lent and rented! This teaches that if Ploni deposited or lent to Levi with witnesses, even if the deposit is 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 a deposit not in front of witnesses. 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. In Bava Basra we explicitly say that If witnesses saw the object deposited, even one who is not a craftsman would not get a Chazakah. Some Rabanan say that Rav Chanina's law and Rava's ruling (that the orphans must return the items) refer to claiming items under the person's garment. Our Sugya refutes them. We asked, and did not answer that they discuss only items under the person's garment! This teaches that Rav Chanina's law always applies. The Halachah follows it.

Rambam (Hilchos To'en 8:3): Something often lent or rented, even if one did not borrow or rent it in front of witnesses, is in the Chazakah of its owner. E.g. if Reuven had a Kli often lent or rented, and witnesses know that it was his, and it was seen with Shimon, and Reuven claims that he lent or rented it to him, and Shimon claims that Reuven sold it or gave it to him for a gift or security, Shimon is not believed. Reuven takes his Kli and swears Heses. Even if Shimon died, Reuven takes his Kli. The Ge'onim say that he swears Heses, for we claim on behalf of orphans.

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 the level of love between them, like the judges assess.

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. Even if something is often lent or rented, he can say that he returned it or 'nothing ever happened' (he never took it from Shimon) if witnesses did not see Shimon give it to him.

SMA (57): This refers to what was said above, that witnesses did not see it with him now. If they did, he could not say that he returned it or 'nothing ever happened.'

Shach (87): Gidulei Terumah asked why the borrower swears Heses that it is not a security if witnesses saw him lend it. I say that if so, it is difficult even regarding something often lent or rented! Since Chazakah helps to take money from one who borrowed or rented, it should exempt from a Shevu'ah! Rather, the Chazakah, or witnesses, only help so that the lender is not Muchzak. Perhaps afterwards he lent and accepted that it will be a security!

Shulchan Aruch (ibid): If something often lent or rented and witnesses saw it now with Reuven, he cannot say that he bought or returned it.

SMA (58): This is when the witnesses know clearly that it belonged to Shimon. If they merely saw that Reuven has a similar Kli to what Shimon claims, we do not force Reuven to show it to Beis Din (CM 297).

Shach (89): This is even if he did not tell the witnesses how he got it. This is even if the witnesses do not recognize it absolutely, only through Simanim, for Reuven fears to say lhd.

Rema: Some say that the witnesses need not see it at the time he claims in Beis Din, rather, they saw it beforehand, and he claimed that he bought it or another reason why he need not return it. He has no Migo to say that he returned it, for we can testify that he did not! Similarly, if it was well-known that he had it, even if there are no witnesses, he cannot say that he bought it, for there is no Migo to say that he returned it, for he fears to contradict what is known. Even if when the witnesses saw it he said 'he owes me such and such ', like he claims now in Beis Din, he is not believed. This is only if witnesses saw before he was claimed from in Beis Din. If they saw only after the claims and dispute in Beis Din, he did not lose his Migo, and is believed to say that he bought or returned it.

Beis Yosef (133 DH Kosav, citing Hagahos Maimoniyos Mishpatim, Teshuvah 32): The witnesses need not see it at the time he claims in Beis Din, rather, they saw it beforehand, and he claimed that he bought it or another reason why he need not return it. He has no Migo to say that he returned it. Even if he will explicitly say that he returned it, he is not believed. If one denied owing, and after witnesses testified that he has he claimed that he paid, he is established to be a denier regarding that money.

Beis Yosef (DH Kasuv): If witnesses saw it only after the claims were made, he was believed at the time through the Migo. Even if we see it later, he does not lose his credibility.

Shach (90): The Rema discusses something often lent or rented. His initial claim that he bought it did not help. Surely he did not return it, for he claimed that it is his! Regarding something not normally lent or rented, he was believed to say that he bought it, so later he is believed to say that it is a security and he is owed such and such for it.

Shach (91): If in Beis Din he also claimed that he bought it, he is believed, Migo (since) he did not need to show it to witnesses from the beginning, and he persists with that claim. Also, if not, no one would ever buy something often lent or rented, lest the seller later claim that he lent it. If he initially claimed that it was a security and he was owed for it, since he admitted that it is not his, he is not believed, for witnesses saw that he has another's property.

Shulchan Aruch (90:12): Reuven is believed about something often lent or rented when witnesses know that it was his and saw it now with Shimon. If witnesses did not see that Shimon has it, he is believed, Migo he could say that he returned it.