I have a few observations which I find keep the consignment store running smothly:
Do not add any new items to the consignment until all current consignment items are sold
If an item remains unsold, remove it and do NOT reconsign, just sell it though your inventory, or if produce, sell it to the shop.
Do not consign the same type of items consecutively.
Example: You consign 6 small paintings. You sell 2 on day one. 3 on day two. No sale on day 3. Go to the store and retrieve the unsold painting do not wait until the day count is up - this will trigger the "Customers are getting bored with your items" message . Do not reconsign it - ever. Consign 3 statues and repeat as above. This seems to prevent the "Customers are getting bored with your items" message. I frequently find that once this message has appeared, it can get stuck and you can never sell anything afterwards no matter how different the items. Reconsigning any item nearly always seems to trigger the message. Your next consignment can be paintings again if you wish. If you place a mixed consignment, then don't include any similar items in the next consignment.
Sometimes consigning similar items consecutively will cause no problem, but, sometimes it does, I think this may be a matter of quantity or quality, but as you won't know untill ypu have triggered the "bored" message it's best avoided.
This is just observation, I have not tested it to destruction, but it works reliably for me.