Both of those convertibles threw me for a loop when I saw them. The Caddy one I figured to be an aftermarket job until I read the description. I guess I'm not used to seeing convertible Caddys that aren't the early 60's models.
The GTO one looks weird too. My first reaction was that it was an edited image. I cannot see the other pictures on that linked website right now as the site requires flash and I'm using a nonflash browser right now. I thought it looked edited because of the way the car was exactly from a side angle (easiest to fake, I'd think) and the background looked funny. However, it did look like the top of the windshield curved properly as you would expect on a convertible (which doesn't stop the car from being a real GTO mixed with the convertible Monaro, I suppose.
However I downloaded that pic and enlarged it. The edges of the trees and all the things that would be blocked by the roof of a real GTO do have the same noise along the edges as things that would not be behind the roof area, and the brick that would have been behind the roof shows real brick texture. The top curve of the windshield frame again shows what looks to be appropriate lighting and curving. I'm not very good at looking for fakes in Photoshop, but those details don't look odd to me.
What does look odd, however, is toward the rear of the car, where the rear window could be, there is a bump up (which could be a bad cut of the roof if this were real) but down the side of the car, when enlarged, I see a lot of smearing. The smearing looks suspect to me. It is about 1/3 the width of the door, and all coming down the side from where the rear window would be.
Additionally, there is some odd protrusion on the top of the front window, that I have no clue what it is. It doesn't look right to be part of the windshield, as it curves out there, but not elsewhere. Here is a picture of the slight enlargement with that protrusion--still kinda hard to see:
