Rasulullaah (SAW) was once passing a grain seller and placed his hand into the heap of grain on display. Feeling that it was wet inside, Rasulullaah (SAW) asked the seller what the problem was. The man admitted that rain had caused the heap to get wet. Rasulullaah (SAW) said, “Why did you then not place the wet grains on top for people to see? The one who deceives us is not one of us.”
[Muslim 284, from Hadhrat Abu Hurayrah (RA)]
Any fault in merchandise on sale needs to be disclosed to the prospective buyers.