Refers to a sales event or discount period, as in a store sale. Not used for the act of selling in general.
デパートで夏のセールが始まった。
The summer sale has started at the department store.
セール中にこのコートを買った。
I bought this coat during the sale.
This department store holds a sale every summer.
The winter clothing sale starts next week.
I bought a cute bag during the sale.
The store gets very crowded during the sale.
セール usually means a sale or price-discount event, whereas キャンペーン can cover discounts but also broader advertising, awareness, or recruitment drives.
going-out-of-business sale; closing-down sale