If you actually wanted to offically sell domain names directly you would have to become a Domain Registrar. This way is very expensive and is a long proccess.
Most people who sell domains are resellers for a registrar, such as enom.
Unless you have thousands of dollars available to invest in becoming a registrar, I would suggest googling around to resell domain names. If you resell you would get a portion of the cost of the domain name.
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