The biggest red flag I see is the price of $15,000, as in US dollars? If this were a true numbers-matching 1971 Mach 1 429 SCJ with 4 speed, I'd expect the price to be triple the asking price at a classic car dealer. This is a blue chip US musclecar and from the limited pictures available on that site it appears to be in excellent condition. Did you have to request the additional pictures, and are they posted on a web site where we could look for other discrepancies? Do you have a pic of the door sticker and the dash VIN, and do they match? If we have the VIN it can be searched on the web to see past sales or other transactions for this car. Although the ad says it has been in the same family since 1975, so not very much would be found if that is true.
The only piece I see missing with a quick glance is the rev limiter on the passenger side shock tower, and quite frankly many of the 429 / 4 speed cars are missing those. The entire condition of the car would need to be checked top to bottom, paint thickness gauge used all over the body panels, underside inspected on a lift (which this facility should have), and so forth. A complete pre-purchase inspection in other words by a mechanic familiar with these cars. Also beware that it could be a scam where the car does not even exist for sale, and the pictures are used from elsewhere on the web. We have helped other 7173mustangs.com users in the past avoid such a scam. When I went to save the four images from the URL provided, they saved as blank GIF files. So I took printscreens of the four pictures, cropped them, and saved them as JPG files. I performed Google Image searches on the four images and only came back with hits for the first pic. That pic was found numerous times on the cars.trovit.com site, which pointed back to the URL that you provided. The sales price is way too low for a blue chip Ford musclecar such as this one, so proceed with caution. The listing says it is from Jan 22, 2019. I doubt very highly this car would still be available at that price for this long, if it is legit.