Commentary on the verse, “Then woe to those who write the Book with their own hands and then say, “This is from Allaah,” to purchase with it a little price! Woe to them for what their hands have written and woe to them for that they earn thereby” [al-Baqarah 2:79]
Question

                I have a question about this verse:

2:79 Then woe to those who write the Book with their own hands, and then say:"This is from Allah," to traffic with it for miserable price!- Woe to them for what their hands do write, and for the gain they make thereby.

What is it that they used to gain by changing the books after they were revealed? Did they literally "sell" these changed books to people and make a profit off of them?.
Praise be to Allah.
      In this verse, Allah warns those who distort the Book and 
      change laws, those who betrayed the trust of knowledge and religion, with 
      loss, punishment and doom on the Day of Resurrection. They distorted the 
      Book by adding and subtracting, and they said that this was from Allah, 
      lying and fabricating. Their aim in distorting the Book was to make 
      transient worldly gains. This is represented in a number of things: 

    1.

    Worldly status by holding onto the leading positions which 
    were enjoyed by the prominent figures and rabbis of the Jews. They knew that 
    if they believed and submitted and accepted the leadership of the new 
    Prophet, they would lose their position that made them look down on people, 
    and their followers would turn away from them to follow the new Prophet, so 
    they erased everything from their Books that foretold the coming of Muhammad 
    (blessings and peace of Allah be upon him) and they concealed what knowledge 
    they had of this matter. Thus they bought transient worldly gain at the cost 
    of the eternal Hereafter. 

    Al-Baghawi (may Allah have mercy on him) said: 

    That is because the rabbis of the Jews were afraid of losing 
    their livelihood and position of leadership, when the Prophet (blessings and 
    peace of Allah be upon him) came to Madeenah. So they tried to prevent the 
    Jews from believing in him. They went to his description in the Torah, where 
    he is described as having a handsome face, with handsome hair, kohl-rimmed 
    eyes and being of average height, and they changed it, writing instead: 
    Tall, blue, with straight hair. When their foolish ones asked them about the 
    description of the Prophet, they read what they had written and said that he 
    was different from this description, so they disbelieved him and rejected 
    him. So Allah said (interpretation of the meaning): “Then woe to those 
    who write the Book with their own hands” [al-Baqarah 2:79]. End quote. 

    Ma‘aalim al-Tanzeel, 1/115 

    Al-Qurtubi (may Allah have mercy on him) said: 

    Ibn Ishaaq and al-Kalbi said: The description of the 
    Messenger of Allah (blessings and peace of Allah be upon him) in their Book 
    said that he was of average height and white, but they changed it to say 
    that he was dark with long, straight hair, and they said to their companions 
    and followers: Look at the description of the Prophet (blessings and peace 
    of Allah be upon him) who will be sent at the end of time; he does not 
    resemble this one. The rabbis and scholars held leadership positions and had 
    an income and they were afraid that if they spoke the truth they would lose 
    their livelihood and positions of leadership, so they changed it. End 
    quote. 

    Al-Jaami‘ li Ahkaam al-Qur’aan 

    2.

    This little price was in return for what they wrote of pages 
    of books. Some of the priests and rabbis of the People of the Book wrote it 
    themselves in order to sell it to some ignorant people and earn money from 
    that. Because of their stinginess and treachery, they deceived them about 
    the word of Allah, may He be glorified and exalted, and they said that these 
    books were from Allah when they are not from Allah. 

    Qataadah (may Allah have mercy on him) said: 

    Some people among the Children of Israel wrote a book with 
    their own hands so as to earn a living from people, and they said: This is 
    from Allah, but it is not from Allah. End quote. 

    Al-Saddi (may Allah have mercy on him) said: 

    Some of the Jews wrote a book themselves to sell to the 
    Arabs, and they told them that it was from Allah, so that they could take a 
    little price for it. End quote. 

    Jaami‘ al-Bayaan, 2/270-271 

    Al-‘Allaamah al-Taahir ibn ‘Ashoor (may Allah have mercy on 
    him) mentioned two other aspects of the worldly greed that led some of the 
    People of the Book to distort their Books. He (may Allah have mercy on him) 
    said:  

    The price meant here is pleasing the common folk, by changing 
    the rulings of the religion for them so that they will be in accordance with 
    their whims and desires, or to claim that they have knowledge even though 
    they are ignorant. So they fabricated worthless books full of stories and 
    simple information so as to pretend that they had knowledge in front of 
    people because they were unable to grasp true knowledge, and they wanted 
    prominence and false leadership. So they fabricated stories and ideas in 
    which there is no true knowledge at all, then they propagated it and 
    attributed it to Allah and His Religion. This is the attitude of the 
    ignorant who seek leadership without being qualified for it, so that they 
    may appear as scholars in the eyes of the common folk and those who cannot 
    distinguish between true and false. End quote. 

    Al-Tahreer wa’l-Tanweer, 1/577 

    And Allah knows best.