RuneScape/Rare Items

Untradeable Items
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
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.