Yes, you can sell it without using Marketplace. You can make a private sale and then change the domain ownership giving it to any user of your choice within Namecheap. If you want to sell the domain to a third party (not to a Namecheap's customer), then you will have to formally transfer the domain to another registrar.We advise you to use our Marketplace to protect yourself from fraudulent activity. We cannot control private deals beyond our billing system and in such cases, we are not responsible for deal conditions to be fulfilled.
If you have any questions, feel free to contact our Support Team.