According to Bonhams, the car was originally sold to Hertz for $3,865. That included optional wheels, radio and power brakes. It served as a rental car until 1967 when it was sold to a private owner who went about modifying the Mustang. That owner added a rear spoiler, a front valence like the GT350R race cars, a roll bar, different wheels, and some auxiliary gauges. At some point, Carroll Shelby bought the car from this owner, and over time several of the modifications were removed in favor of original-style pieces, notably the front valence and the rear spoiler. The roll bar remains, and it still has some auxiliary gauges, though.
The car is coming straight from the Shelby collection, so the only owners have been Hertz, the buyer from 1967, and Mr. Shelby himself. On January 18, you could be the next owner, but you'll be paying quite a bit more than that sub-$4,000 price tag. Bonhams expects the car will sell for between $150,000 and $200,000.