The Trade Hold is a security feature to deter fraud in Rocket League. A trade hold prevents an item from being traded for a 7 days after it is purchased. However, this doesn't mean the key can't be used, it just can´t be traded with another player.
If the key is used unlock a Crate within those 7 days, the trade lock will be passed down to the item.
Note: Trade holds will not be lifted under any circumstance.