In Wisconsin, a demonstrator car is considered a used car by law. See Wis. Admin. Code Trans 139.02(20). Therefore, a dealer may not sell a consumer a demonstrator and represent that it is a “new car.” Doing so may violate a consumer’s rights.
I’m an attorney, not an auto repair shop, so I don’t have specific answers to this issue. I did find a couple of books, though, that talk about signs of a wreck. They are written by Danny Wyatt, who ran a body shop in North Carolina for years. He wrote 2 books on the subject: “Collusion Collision” and “Signs of a Wreck.” You can see more about them at http://csiofnc.com/collisioncollusion-signsofawreck.html. If you are buying a new or used car, it may be worth the look to prevent being tricked by a car salesperson or car dealer that has taken a wreck or salvaged car and fixed it up to look brand new. A few ounces of car fraud prevention may be worth a many more pounds of cure.
If you buy a new car that has 900 or so miles on it, there is a good chance that this car is not new. If the car dealer previously sold this car—that is that another buyer signed a purchase agreement and drove it off the lot—and then the prior buyer gives the car back for any reason, the dealer should then sell this car as a used car. The price differential should be significant. Chances are too that the prior buyer noticed problems with the car, and that may be the reason for the prior buyer returning it. Tell tale signs of a used “new car” tend to be high mileage and problems very early on. Before you consider buying a “new car” with high mileage on it, get representations from the car dealer, preferably in writing, about why the mileage is so high, whether it was ever sold before and strongly consider whether you really want that car for the “new car price.” Generally, a dealer that sells a used car as new is in violation of your consumer rights.