@FBM,
Thank you for your reply, FBM.
As far as I know 'on sale' has something to do with 'lowered price',
while 'for sale' is related with 'the act of selling something'.
So, if something is on sale, I figured, the price of it is reduced
and if something is for sale, I thought, the ower of it is trying to sell it and
whether the price is lowered or not isn't clear.
With that in my mind, I made up a sentence :
The books are on sale whenever they are sold,
whose meaning I intended was,
The books are sold at a lower price every time they are sold.
From the comments of the people here, I could sense the sentence I made up doesn't make sense.
Now, would it be okay to conclude that 'on sale' is not referring to any lowered price?
http://www.yourdictionary.com/on-sale
From the site above, its meaning seems to vary depending on the region.
Would the sentence I made up possibly be considered okay in the US,
not in UK?
How would 'on sale' generally be understood?
I'd really really love to see your comment.