I paid for a full OPC inspection (111pt check) of my 911 before I sold it. Also included a service too, but then I also priced both of these into the asking price so I didn't lose out
Thing is, 95% of buyers don't know or don't care: They just want a cheap Porsche. There's no point spending money out on the car if you just want to get shot of it, and then what happens if you do find out there's an issue? You either have to disclose it in the ad and reduce the price, or hope that someone comes along and doesn't ask the right questions or want their own inspection.