Everything must go! Gracie is holding a sale now too. The sweets items will be less expensive than usual, but fewer items will appear as they "sell-out." In City Folk I was always poor and had to wait for the sale to get what I could from Gracie, lol. The island has changed that. I saved up for each set pretty easily before the sales began.
As the others said, the sweet collection is discounted. I've never been successful getting the whole collection at the sale prices. I buy what I can and will get the rest next year.
The card furniture is not discounted, but is only available during the sale.
There's also clothes and furniture that can only be bought during sales! Certain shirts, like the waistcoat, and furniture like the Wedding cake for example.