Therefore, write down this poem and teach it to the people of Israel; put it in their mouths, in order that this poem may be My witness against the people of Israel.

Rava says: With regard to the mitzva for every Jew to write himself a Torah scroll, even if a person’s ancestors left him a Torah scroll, it is a mitzva to write a scroll of one’s own, as it is stated: “Now, therefore, write for yourselves this song and teach it to the children of Israel” (Deuteronomy 31:19). Abaye raised an objection to him from a baraita concerning the king’s Torah scroll: And he writes himself a Torah scroll for his sake, so that he does not beautify himself with the Torah scroll of others. Read precisely, this indicates that a king, yes, he is included in the halakha not to have a scroll inherited from his ancestors suffice, but an ordinary person is not.

And Rabbi Yehoshua bar Abba says that Rav Giddel says that Rav says: One who purchases a Torah scroll in the marketplace is akin to one who snatches a mitzva in the marketplace, as the proper manner in which to perform the mitzva of writing a Torah scroll is to write one for himself. And if he himself writes a Torah scroll, the verse ascribes him credit as though he received it at Mount Sinai. Rav Sheshet says: If he emended even a single letter of the Torah scroll, thereby completing it, the verse ascribes him credit as though he had written it in its entirety.

That is that He commanded us that every man among us should write a Torah scroll for himself. And if he writes it with his hand, Scripture accounts it to him as if he received it at Mount Sinai. But if it is impossible for him to write it, he should buy it or hire someone to write it for him. And this is His saying, "write for yourselves this song " (Deuteronomy 31:19). And it is not permitted to write [only] sections. For when He said, "this song," He intended the whole Torah, which includes this song (of Haazinu). And the language of the Gemara (Sanhedrin 21b) is, "Rabbah said, 'Even if his ancestors left him a Torah scroll, it is a commandment to write a scroll of his own, as it is stated, "write for yourselves."' Abaye raised an objection to him (from a baraita concerning the king’s Torah scroll), '"And he writes himself a Torah scroll for himself, so that he does not become proud from the Torah scroll of his ancestors." A king, yes, but an ordinary person, no!'" And the answer was, "No, [the ruling of that baraita] was only necessary [to teach that the king is commanded to write] two Torahs; as it is taught in a baraita, 'And he must write two Torahs for himself.'" This means to say that the difference between a king and an ordinary person is that every man is obligated to write a Torah scroll, whereas a king, two - as it is explained in the second chapter of Sanhedrin. And the regulations of this commandment - meaning to say, the writing of a Torah scroll and its stipulations - have already been explained in the third chapter of Menachot and in Shabbat, Chapter 16. (See Parashat Nitzavim; Mishneh Torah, Tefillin, Mezuzah and the Torah Scroll.)

It is a positive commandment for each and every Jewish man to write a Torah scroll for himself, as [implied by the commandment (Deuteronomy 31:19)]: "And now, write down this song for yourselves," i.e., write down the [entire] Torah which contains this song. [The basis for this interpretation is] that the Torah should not be written passage by passage.
Even if a person's ancestors left him a Torah scroll, it is a mitzvah to write one himself. If a person writes the scroll by hand, it is considered as if he received it on Mount Sinai. If he does not know how to write himself, [he should have] others write it for him.
Anyone who checks even a single letter of a Torah scroll is considered as if he wrote the entire scroll.

A king is commanded to write another Torah scroll for himself, for the sake of his sovereignty, in addition to the scroll which he possessed while a commoner, as [Deuteronomy 17:18] states: "And when he sits on his royal throne, he shall write...." This scroll should be checked against the scroll in the Temple Courtyard by the Supreme Sanhedrin.
The one which he possessed while he was a commoner should be placed in his storage chambers, and the one that he wrote - or had written for him - while he was a king, should be with him at all times. When he goes out to war, his Torah scroll should be with him. When he returns, it should be with him. When he sits in judgment, it should be with him. When he dines, it should be opposite him, as [Deuteronomy 17:19] states: "And it shall be with him and he shall read it all the days of his life."

Laws of Torah Scrolls

2) Nowadays the commandment is to copy the five books of Moses, the Mishna, and the Talmud with its commentaries. [These books] can only be sold [in order to gather funds] to study Torah or to marry a woman.