I had an odd experience with a Canon lens I bought on a Boxing Day sale. The lens was almost two thirds off the regular price. I got the last one, but the box was very damaged, I went back two more times during the sale and they never got any more stock. I made the mistake of not opening the box to look at the lens for two months! When I did open it, I found that the lens had been used, not repackaged properly and the lens had obvious scratches and was not clean. When I took it back, the guy at the customer service said no way I can do anything for you you've had the product for over 15 days. I asked to see the manager and they offered me an instore credit. I asked to see the store manager and he allowed me to exchange it for a new one.
So this was in good faith on their part, though I gave them new money for what I thought was a new lens, which someone obviously used for a week or so and took back during the busy Christmas rush, and was put back on the shelf as "new".
The experience wasn't that great, do to the attitude of the customer service guy, who still was treating me like I was BSing them. But they did give me a complete exchange, so I am satisfied.
It pays to have them open up any purchase, and check it for use or damage. I notice they routinely do this for TVs for the last couple of years. Customer service really need to check returns as well for use.