First of all you need to defeat the Elite 4.
After that keep watching for the TV (any TV) to flash (meaning there is news on the TV that might interest you). When it does flash go read it, if you are lucky, it should say something about Pokemon News and that the clearout sale is coming in how many days. Usually if you check the TV everyday, when you first see the "commercial" for the clearout sale, the game gives you 2 days to "prepare" for the sale :) On the day of the sale, just make sure you talked to the TV then head up to the Lilycove department store and you should see a different NPC up on the rooftop that sells unseen items.