Sell is a verb and sale is a noun:
– I’m going to sell my car and buy a new one.
– She’s selling bottles of water at the football game.
– Yesterday I sold all of my old college textbooks on the internet.
– The bookstore is having a Christmas sale – everything is 20% off. (20% off = 20% discount)
– The sales of our new product are slowly increasing.
– This watch is normally $100, but it’s on sale for $70.
(on sale = there is a discount)