This is how I interpret this rule. Correct me if I am wrong.
Weeks 1 to 8: any sim can sell any object on any day at the consignment store.
Week 9:
Sim 1 may sell 4 items on Sunday, 4 items on Monday, 4 items on Tuesday, and so on. Sim 1 may sell a total of 28 items in week 9, and all must be the same type.
Week 10:
Sim 2 may sell 4 items on Sunday, 4 items on Monday, 4 items on Tuesday, and so on. Sim 2 may sell a total of 28 items in week 10, and all must be the same type.
Week 11:
Sim 3 may sell a total of 28 of the same type of item, as above.
Week 12:
Sim 4 may sell a total of 28 of the same type of item, as above.
Do we need to cancel any of the 4 items if they are unsold after the day they were put up for consignment?
No. But, if one item remains unsold then the following day you would only be able to list 3 new ones.
I interpret this to mean that Sim 1 may put 4 items up for sale on Sunday of Week 9. Three of them sell, one does not. Option A: Remove the unsold item from the consignment store, and put 4 new items (or three new and the unsold one) up for sale on Monday. Option B: Leave the unsold item at the consignment store, and put 3 new items up for sale on Monday.