Can I sell music and videos online? How?

Make a deal with the people who own the rights to that music and get a written contract signed by them saying that you can.

The First Sale doctrine allows you to sell the CD or DVD that you own to whomever you wish. You can also buy these items from others and sell them on-line. That's the underlying basis of companies like Amazon. However, unless you have a license to duplicate the music or video, you can not make copies of the work in any format (physical or electronic) and sell those on-line. That would violate copyright laws. The exception is if the work is no longer covered by copyright which generally is anything created prior to the 1920s. ASCAP and BMI handle a lot of the copyrighted music; the individual production companies or film studios do the licensing for their movies.

You can sell "used" original copies of music and videos. You cannot make copies (MP3s) and sell those - as your copying violates the rights of the original producer of the music or video - COPYRIGHT INFRINGEMENT.