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for sale vs. on sale
what's a big difference between these two expressions?
are these two expressions interchangeable?
can someone check the following sentences to see if I am understanding the two expressions correctly?
thank you!!

1. this product is not for sale. It's a sample for display.
=this product is not on sale. It's a sample for display.

3. This product is on sale so you can get a 20% discount.
=this product is for sale do you can get a 20% discount.
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In the US, "For sale" = available to be sold. #1 is "not for sale" because it's a sample and they don't want the customer to buy it.

"On sale" is usually an indication of a special price. #3 is "on sale" because it's a lower price than usual.
Comments  
Students: Are you brave enough to let our tutors analyse your pronunciation?
in my dictionary, "for(on) sale " menas "available to be sold".... so I am just wondering "on sale" could be possibly used as a meaning of "available to be sold" ??
Hi Bn77

This is what Cambridge learners dictionary says:

on sale UK
available to buy in a shop

on sale
mainly US (UK usually in the sale)
reduced in price

So yes, 'on sale' would mean 'available to be sold' in the UK.

PBF
I think" for sale" means the "purpose"; not for display or giving out for free." On sale" means something is being sold there and then.I stand to be corrected
Students: We have free audio pronunciation exercises.

For sale is used for a regular sale while no particular discount is given, but on sale is used to offer a discount.