Responsa על בבא קמא 228:10

Q. A book was stolen from A and sold to B for one mark. It was subsequently stolen from B and sold to C for the same price. Both A and B recognized the book in C's possession.
A. If at the time of the theft from A, the fact was reported in the town, and if A does not usually sell his books, then A is to pay one mark to C and reclaim his book. B may look for the man who sold him the book and sue him; if he cannot find him, his money is lost.
SOURCES: Pr. 88.

Q - Forced converts, after having reverted back to Judaism, reported that while they were still among the Gentiles, they saw the body of A who was killed, lying in front of his house. It was discovered that these converts, prior to their return, partook of non-kosher food, purely out of lust. On the basis of their testimony, is A's wife permitted to remarry?
A - Although these forced converts formerly partook of non-kosher food, they are believed to be telling the truth, since they have now returned to Judaism. Therefore, A's wife is permitted to remarry.

Q B discovered that the horse he bought from A is injured. He claims that the horse must have been injured while in A's possession, that the purchase was a mistake and demands, therefore, that the contract be rescinded. A, on the other hand, claims that the horse was in perfect condition when he delivered it to B.
A. Even if B had not, as yet, paid for the horse, the burden is upon him to prove that the horse was injured while in A's possession. If he cannot produce such proof, A is to swear that when he sold the horse to B he did not know that the horse was injured. If he takes such an oath he is entitled to collect his money from B.
SOURCES: Pr. 575; L. 122; Mord. Ket. 200; Tesh. Maim. to Kinyan, 7; Mordecai Hagadol, p. 165d; cf. Sinai vol. VI (1942) p. 221.