One of my clients got in some trouble recently over a "'No refund on sale items" sign. A lawyer who is one of their customers told them that they were "misrepresenting rights available to consumers."
So if you wish to put up a no refund sign, use one from your state consumer affairs sign such as this example from VIC above.
One worrying point is that many merchants do sometimes unload at significantly reduced prices goods that are defective. They admit that they are flawed and so sell it with an AS IS and NO Return clause. I doubt legally this is correct. You may want to check this situation if you do it a lot. If you get an answer, let me know.