Newer event items have supposedly been deemed non-tradeable to reduce the number of high costing items.
Masks
Masks are the oldest rare items, thus are the most expensive.
Santa Hats
A Santa Hat.
Santa Hats come from the year 2000's Christmas event; where at 12:00 AM hats began appearing all over the world. Santa Hats have been said to go for several million gold a piece.
