Actually, unless it is with the intent to defraud it is not illegal...If you reset it and then tried to sell the car as original mileage as shown on the odometer at any time, it would be illegal.
TITLE 49 - TRANSPORTATION SUBTITLE VI - MOTOR VEHICLE AND DRIVER PROGRAMS PART C - INFORMATION, STANDARDS, AND REQUIREMENTS CHAPTER 327 - ODOMETERS
HEAD
Sec. 32703. Preventing tampering
STATUTE
A person may not - (1) advertise for sale, sell, use, install, or have installed, a device that makes an odometer of a motor vehicle register a mileage different from the mileage the vehicle was driven, as registered by the odometer within the designed tolerance of the manufacturer of the odometer; (2) disconnect, reset, alter, or have disconnected, reset, or altered, an odometer of a motor vehicle intending to change the mileage registered by the odometer; (3) with intent to defraud, operate a motor vehicle on a street, road, or highway if the person knows that the odometer of the vehicle is disconnected or not operating; or (4) conspire to violate this section or section 32704 or 32705 of this title.