Rule:
Purchasing items from stores or Buy Mode is fine, but homes cannot be altered in any way—no moving, deleting, upgrading, or leaving anything on the lot at moving time.
Clarification:
You cannot sell furniture or anything in the furnished home, even if you don't need it.
You cannot move any furniture, even if it annoys you.
You cannot upgrade any item that comes in the furnished home.
You cannot leave any items on the lot at moving time - this is to prevent you from inflating the home value.