I have bought and sold on CU with various members.
When I have currency for sale I list it here and at CU. In one instance I had 3 dealer responses within 2 hours and the item sold. Most of my items have ended being sold over on CU than here. There is a larger network of currency dealers / collectors over there.
To circle back -- It depends on the item...meaning more rare / hard to find items move fast. Good luck.