Tips on Buying Cars: Demos, tire rotations, dealer demo

Question
What is considered a dealer demo? I bought a 2008 Mariner represented as a demo driven by a dealer employee. It has 22,000 miles on it. When I got home I discovered that none of the shceduled maintenence had been documented in the maintenance guide. I ordered a car fax history and discovered the car was sold to a rental company in 2007 and registered in Washington as a rental car then sold a few months ago at auction. The problem is that the complete lack of a record of service voids the warranty.

Answer
I cannot speak for WA but in my state, dealer demos are always sold as new, never titled vehicles.  You were definitely sold a used car.

The lack of maintenance documentation in the book does not void your warranty.  Very few people ever fill those out.  A manufacturer can only deny warranty coverage for maintenance reasons if the lack of maintenance has caused a component to fail and even then, the denial is limited to the failed component.  They must also bear the responsibility of proving the cause of such a failure.
I would however, take the vehicle back and demand that they perform whatever maintenance is needed to date.  At 22,000 miles however the only things likely needed are oil changes and tire rotations.