Gold

Grand auction halls... where the most rare and precious items are sold, where the wealthiest and the strongest compete with each other to win them 

To hold such an auction and invite the powerhouses of the Kingdom, then it must contain many types of strong beasts hides, Weapons, Natural resources, Tough minerals, and many more....

It is one of the most important practices on the continent and in almost every city there is at least a small hall, especially the auction halls in the ancient Ducal cities are the most prosperous and rich in treasures

But those ancient auction halls often take a year or more to collect enough resources to call for an auction 

And of course, the ultimate number one is the Capital Auction Hall, which is an event that takes place every five years and brings together all the nobles of the Kingdom, no matter what they are doing, they must leave everything aside and go to have look there.