Black Market
From UE Wiki
About Black Market
Every time you find a random item whilst playing U-E, it will get put in your inventory in your Home. These items can either be used or place in the Black Market for sale.
Selling in the Black Market
When you put an item for sale, you can either charge Money or Gold for them. It will be placed for sale for a total of 30 days. If no one buys it, then it will get returned to your inventory.
