If I found a crate in Team Fortress 2 and open it with a key, can I trade or craft the item stored inside?
Yes, crate-opened items can be crafted or traded.
The only items not craftable or tradeable (excluding event or limited edition items) are those bought directly from the store.
If items bought from the store could be traded and crafted, there is a possibility of massive, massive inflation on the item economy. Hence, the ban.