If you have the chance to see in person, lok underneth to get an idea if it has been offroad or thru the rocks.... you'll be looking for the normal scraps on teh t-cas skid and gas tank skids, check the lower controls are brackets to see if they occured any damage and then check the front & rear pinions. Dont be afraid to grab the driveshafts & u-joints as well to see if theres any play in them. Most people dont even consider any of those since its at the dealer lot.
I recently did that about amonth ago in the rain, look at a Ram 2500. The dealer thougth I was crazy, since it was raining out & I was crawling around under the truck, but I knew what key things to look for since I knew I would be towing with this.