Am I able to sell relics that I have no need for? And if I can, how do I do it?

Yes. Check out Montak's Antiquities under "realm".

You can also sell them to players. Go to your den, and besides all those info next to the avatar, you will see the small list with 'Send Mail' 'Send Mushrooms' and such. Almost at the bottom it will say Market den. If you click that, you have to scroll down. Soon you will see how you have to sell stuff