Refurbished could mean many things. It applies to all manufacturers, not just NAD. It could be overstock, demo, or even defective equipment that has been returned and repaired. The defect could be electronic, or it may only have been cosmetic, a new faceplate for example. If it has been manufacturer repaired, then it has been repaired to new condition, and it's at a discount price. I think it's a great deal, although I would want a warranty to back that up.
If you look inside a NAD and compare it with its competitors, Rotel for example, it does look a little cheaply made. However, there are many happy NAD owners who have had years of enjoyment from their equipment. If you're concerned, just buy warrantied products from reputable dealers.