Ronaldo, who is moving to Real Madrid in a world record £80 million move this summer, is the only previous owner of the Carrera 2 Tiptronic S.
He bought the car in 2007 and then sold it to a dealer who has now put it up for sale at £69,995.
The advertisement appeared as the 24-year-old, who this week discovered he would not face charges after writing off his Ferrari in Manchester, prepares to relocate from England to Spain.
Under the bonnet of Ronaldo's former black supercar is a 3.8 litre engine with 355bhp allowing acceleration of 0-62mph in just 5.4 seconds.
Auto Trader describes the model, which has done just 16,000 miles, as having upgraded leather seats, satnav, 911 Turbo alloy wheels and a tracker.
The advert says: "While he is the only registered owner of the car, Ronaldo doesn't currently possess the Porsche with the supercar being sold by a dealer with the footballer's name signed on the log book."
The dealer – RCM Trade Sales – states: "Here's your chance to get your hands on Cristiano Ronaldo's Porsche 911
! The 355bhp convertible is fully loaded with satnav, electric windows, leather seats, 911 Turbo alloy wheels and Cobra tracker. This awesome handling supercar has just 16,000 miles on the clock and was serviced 50 miles ago. 0-62mph in 5.4 seconds, top speed 177mph. Perfect for roof down summer driving. Log book is signed by Ronaldo. £69,995 or offers above £69,995."
Ronaldo has owned a host of supercars since his move from Sporting Lisbon to United five years ago.
His cars have included a Porsche Cayenne, Bentley Continental GTC, Audi R8
and Bentley Continental GT Speed His Porsche Cayenne sold instantly after being advertised on Auto Trader last year and his Bentley Continental GTC also sold quickly after being sold through Auto Trader. Supercar Driving Experiences