The 245th mitzvah is that we are commanded regarding the laws of buying and selling, i.e., the ways in which purchases and sales between the buyers and the sellers become legally binding.
The Torah taught about one method in G‑d's statement (exalted be He), "When you sell something to your neighbor, [or buy something from your neighbor's hand...]" Our Sages said, "[The word 'hand' teaches that the sale] refers to something which can pass from one hand to another," i.e., meshichah [physically moving the object].
It is explained that in Biblical law, transfer of money is sufficient to complete the transaction, and meshichah is necessary only by Rabbinic decree, as is mesirah [giving the vehicle of control, e.g., the reins of a horse, to the buyer] and hagba'ah [lifting the object].
The Gemara explicitly says, "Just as our Sages enacted a requirement of meshichah in order for a sale to be valid, so too they required meshichah in order for a watchman relationship to become valid." It is therefore clear that the requirement of meshichah in buying and selling is of Rabbinic origin, as explained in the relevant place.
However, other methods of acquiring land, etc., i.e., by means of a document or chazakah are traced to Biblical verses [and are therefore of Biblical, not Rabbinic, origin].
The details of this mitzvah — i.e., the manners of finalizing a sale in each category — are explained in the 1st chapter of tractate Kiddushin, the 4th and 8th chapters of Bava Metzia, and the 3rd, 4th, 5th, 6th, and 7th chapters of Bava Basra.
