In order to activate the Rooftop Sale on the roof of the Lilycove Department Store you need to have beaten the Elite Four, and it appears randomly. A helpful tip to find these random occasions you need to check in with the Pokemon News Station.
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4: Berry master is in Lilycove (blend the best berries), Rooftop sale in Lilycove (buy worthless decorations for base), Energy Guru sale in slateport (half price protein/iron/etc.!), and appreciation day at the mauville game corner (improves odds in all of the games)
After you beat the elite four, wait till the tv in a random person's house flashes. It could be the rooftop sale. Then if it is wait two days and watch the tv again and go to lilycove department store!
It isn't about the date and stuff... but it will say on the tv sometimes....
They are only available at the special sales in the Lilycove Department Store. They are held at the rooftop. To find out when those sales are occurring, watch the televisions inside of houses. It will tell you how many days are left until the sale occurs.
They arent for sale, you can only find some.