A copyright is automatic. In other words, as soon as you create a new beat, you hold copyright for that sequence. Copyright gives the holder the exclusive right to sell the work, which means that you cannot legally sell beats if you do not hold the copyright for them. If you hold copyright, it is up to you to decide if you want to sell the copyright along with the beats (many record label contracts require artists to hand over the copyright) or issue a license.
A copyright holder has the exclusive right to copy or reproduce a given cultural work (such as music, drawings, photographs, books and poems) in any way, as well as sell, promote, perform and distribute it. The person who creates an original work legally owns the copyright to the creation has the option to sell the rights. Many artists and businesses also decide to register works with the U.S. Copyright Office, which charges a fee and issues a certificate. The certificate is simply further proof of ownership.
You can sell beats to someone and keep the copyright or sell the beats along with the copyright. If you keep the copyright, the buyer can use the beats for personal use, but cannot copy or reproduce them, promote, play, adapt (or remix), distribute or perform them. You keep the copyright unless you and the buyer specifically arrange for a copyright transfer, which involves a contract and notifying the Copyright Office (if the beats are registered).
Many artists opt to issue licenses for their work. A license specifies that the user has permission from the copyright holder to use a creation in a specified way and a specified place for a specified amount of time. If a buyer wants to use the beats for purposes that are not personal use and consumption (sampling on a recording or during a live performance, or use on a sound track), you can issue a license for that particular use. Usually, you can charge the buyer for the license.
You can also opt to sell the beats and the copyright to them regardless of whether you hold a Certificate of Registration from the Copyright Office (the buyer may ask for a certificate, however, to prove ownership). The Copyright Office advises getting a lawyer to write up a contract agreeing to the terms (and payment) and officially transferring the copyright. While keeping or selling a copyright is ultimately the creator's decision, many record labels and other industry players require that artists transfer copyrights to the company before working with the artist or the artist's work.