1. The fruits are on sale.
2. The Fruits are for sale.
(What is the difference between them?)
3. The product is on sale, 30% off.
4. The product is 30% off.
5. The product is sold 30% off.
6. The product is 30% off on sale.
(Are they all grammatical?)
1. lower than usual price
2. at whatever price
In 3 through 6 it is the price of the product, not the product, which is 30% off.