Used carbon is generally a poor idea unless you really know what the in depth history of that fork is. I wouldn't want to buy someone's used carbon because I don't know what has happened to it and I cannot guarantee it is safe to ride on.

Buy a new one, support your local shop or at least Scrodzy who runs a small shop which for some may not be local but specializes in those fixed gear and single speed bikes the kids are all talking about these days.