How do people sell domain names?

[rambo6376]

I'm sure many people think this is a silly question but I am a newbie and I honestly dont know how to sell a domain to someone. I didnt know how exactly to search for it. I dont mean like making a domain reseller site. I just mean how do I sell a single domain that I own? If someone offers me $20 for my domain do they pay me $20 and then pay a registrar the fee required to change the owner and re-register? Last question: if someone purchases a domain, does the expiration date remain the same?