1. It is cheeper to buy things on a bargain sale.
2. It is cheeper to buy things on sale.
(What is the difference between them?)
3. The apples are on a bargain sale today.
4. The apples are on bargain sale today.
5. The apples are on sale today.
(Are they all grammatical? Which ones are right? )
6. The apples are for sale.
7. The apples are not for sale.
(Can we see those expressions as in #6 and #7 at a shop?)
All are grammatical. One through five essentially mean the same thing, about being "on sale" or "bargain sale".
Six and seven would not be seen at a shop. If the apples are displayed in a shop, it's assumed they are for sale. If they are not to be sold to customers, they would not be on display. Both statements are grammatical, though.