Open your bag using F2 or the icon in the shortcuts bar.

Use the left tabs to browse between city and sports items. Use the top tabs to browse the various types of items.

Items are organized by category (head, torso, hands,...), then by date of purchase (from the newest to the oldest).

Click on a slot near your avatar to display items from this category only.

On the bottom right corner, you'll find a button to get Gold and a button to sell items for some Silver.