I think you are right about the seller. He may or not have known it would not fit, but it's "buyer beware" if you know what I mean.Recently I bought an x pipe from a guy who said it would fit a 2013 GT. I went to the shop to get it installed and they said that it doesn’t fit, something about the flanges where the x pipe would replace the factory H pipe wouldn’t adapt to the x pipe. He said that the exhaust is different on the GTCS but I went online and couldn’t find anything about the exhaust system being different than a regular 13 GT. Does anybody know if the CS has a different exhaust system than the GTs? I’m thinking the guy who sold me it lied about what years it fit.