In order to unlock the secret shop you must completely fill the collector's book in the game (this requires at least two playthroughs). Upon completion, speak to the man dressed in purple at the library in the Daath church to receive Jade's "Item Collector" title. Speak to the man again with the title equipped and the shop will appear near the Inn in Grand Chokmah.

Unlockable - How to Unlock

Secret Shop (Brillante) - Complete the collector's book