Room 4/29 is a plaza full of damaged market stalls. When midnight comes around, the stalls transform, as if repaired, and become laden with goods. Sounds of people trading fills the air, even as no actual people walk about. The area remains this way till sunrise.
Adventurers can trade for basic goods, and sometimes rarer fair. Vendors, though unseen, actively bargain, though they don't trade in coin. Instead, it's like goods for like goods, or else an adventurer can trade something a bit more intangible, such as their True Name, a year of service, their voice, etc.
If someone takes an item without trading for it, the item becomes cursed at sunrise.