If you have several domain names – or even just one – that you aren’t using, you might plan to just let them expire. You can certainly do that, but someone else might want them, and you could sell them rather than let them lapse. If you do that you’ll get the money for them, instead of allowing the company you bought the domain name from to get it back.
Selling a domain name isn’t that difficult, but how you do it may depend to some extent on who you bought it from. There may also be fees or commissions involved, but you can work those out with the buyer. There are also domain name auctions. You could put the name out there and see if anyone was interested in bidding on it.
Most domain names don’t bring a lot of money, but there are some that are very popular. There are also names that you might not think have much value, but someone else with a great business idea might completely disagree. It’s worth checking into, because it could mean money in your pocket instead of just saying goodbye to an expired domain that you aren’t using anymore. Don’t assume there’s no value to it until you see if others are interested in it. If there’s a lot of demand for a particular domain name, you might also want to hang onto it, renewing it for a while and selling it later.
You’ll need to analyze trends for this, though, in order to determine whether the value of that domain name will keep rising. If it will, holding out for more money later makes sense. If there’s a good chance that the name won’t stay as valuable you would be smart to sell it and get the money for it while there’s still a high level of interest.