LEV968 It is a positive commandment to render judgment on matters of buying and selling by the rule that the Torah ordained about it. This is to say that there are situations where a transaction between the seller and buyer remains valid, and situations where it has no validity; and an obligation lies upon us to judge between them according to the instruction of the Torah. A specific commandment is given about this in a specific portion [of Scripture]: And if you sell something to your fellow or buy from your fellow's hand, etc. [this verse] – – because it [ trade] is a constant thing among human beings, without which it is impossible for people to live even one day. Land can be acquired as one's possession through money, a document, or demonstration of ownership; and movable goods or a domestic animal, by drawing [it to oneself] and all the more certainly by lifting it, and so forth.