How to sell Fortnite brick?

Excuse me, I'm a bit confused about something. I've heard of people talking about selling Fortnite bricks, but I'm not entirely sure what they mean. Could you please clarify what a Fortnite brick is and how one would go about selling them? Is it some sort of in-game item or currency? And if so, how does one determine its value and find potential buyers? I'm really interested in understanding this concept better. Thank you in advance for your help.