Odometer fraud is the act of setting back the mileage meter so that the car appears to have been driven less than it has. This is highly illegal but it is done more than you may think. Changing the odometer is not difficult and some take advantage of that. Service receipts and oil change stickers all mention the mileage. See that they match what the odometer says. It is not only private sellers but some used car lots will set back the odometers as well.
If possible, have the car checked by a trusted mechanic. They pretty much know exactly what to look for which will give away the actual mileage of a car. If you cannot do that then it is best to go to the DMV and have a title search done. Based on the VIN of the car, the entire history of the car is available. The DMV also has a website on which you can do a search based on the vehicle identification number. This is a free service and it is best to take advantage of it. Not only will it tell you if the odometer has been altered, but it will tell you any repairs that have been done to the car and if it has floor or fire damage. Making sure you do not fall victim to odometer fraud can save you a lot of money and frustration in the future. Take the steps necessary to avoid it.