Bruce, -for sale.....available for purchase -on sale.....1) for sale ...................2) available for purchase at a reduced price

The shop is on sale (selling items at a lower or discounted price) The car is for sale (being sold, with or without a discount)

If something is "for sale" (I have a car for sale), then it is available to buy with no reference to price. "On sale" can also mean this especially in phrases like "The new iPad went on sale this morning..." but it also means 'at a reduced price' as in "bananas are on sale this month".