Some of the D600's are being sold as "refurbished". I'm not sure what that really means. It might simply be a ploy to sell off the cameras for a cheaper price, and with a reduced warranty period of only 90 days.
It takes approximately 3000 shutter clicks, preferably without changing lenses, to test for excessive sensor dust. For us that is not a problem. We shoot in excess of 3000 exposures in a typical work day. We decided to pick up a refurbished model to use as another backup unit in our inventory. Because of the time limited warranty we wanted to immediately determine whether or not our D600 had the sensor dust problem, fully expecting that it did. Sure enough, after 4000 shutter clicks we had plenty of dust specs in the upper left area of the image frame, a sure sign of the problem. So off the camera went to Nikon Service along with a photo showing the sensor dust spots.
Bottom line is the camera was repaired by Nikon Service with the replacement of a new shutter mechanism, all covered under the warranty. Now we have a really good backup camera, which we picked up at a very low price.
I am not recommending that others do what we did. It did involve taking a chance. In our situation it was well worth the effort. For someone else it might not be. But as a backup camera it turned out to be a really good deal.