For items to be listed at the broker there are two ways. Buy a bulliten board as Icidi said, then:
1 - Stay in you room and anything in your bags will be listed after you assign a price and put them up for sale.
2 - Put items in your room vault. From inside the room, right click on the door > Access > Vault. There you will see 2 slots to put things. The smart seller will place 2 bags or boxes in those slots to open up even more space. This way does not require you to stay in your room, but items will be removed fro mthe brokers list 24 hours after you log off.
You assign prices to your items through the bulliten board.
Good luck!