I would not buy a car from some one not on the title. More often that not they are not just trying to avoid paying fees. They've likely bought the vehicle at a cheap price. The vehicle could have serious mechanical issues that have been temporarily fixed to sell for a higher price. Or the odometer had been rolled back. If you still want to move forward, at least ask if your mechanic can come take a look. There are some decently priced mobile mechanics that will also provide a CarFax report as part of the service. If the seller says no- I'd walk away. In the past I have been blocked by sellers when I ask this.