With the change of the ingame's Item Shop we not only rearranged things a bit and added a search - we also added a little button to the menue: "Special"

Every day, one item from the list of the most popular, best selling items (e.g. mounts, potions, daily quest reset stones, etc.) will be randonly selected and is on offer for half-price! (rounded up)

These items are highlighted by a pink frame.

Link: Official RoM-Forums