Does a person bear the sin for his friend watching movies or listening to songs after he bought him an internet cable to operate the satellite dish which has Islamic channels?

As long as the questioner does not definitively know or strongly suspect that the person will use the cable for something forbidden, there is no sin upon him for selling it to him. If the buyer uses it for something forbidden, the sin is solely upon him. However, it is advisable to counsel him and forbid him from evil if he is seen committing the sin.