You can now buy and sell CS:GO Knives in CS:GO Skinbanking! When we announced CS:GO Skinbanking, we promised that this was coming soon -- and soon is now!
Knives are a different beast than weapon skins, so this is still early stages. We'll be continuously improving the way we handle knives, as well as increasing the range of knives that we accept.
Check it out!
Do not click that accept button yet! Actually read them before doing so.