RC:
1. Porsche will always favor cars bought from official dealers regarding warranty issues and a possible grace period after the warranty expires
Porsche would end up in major trouble (e.g. lawsuits) if they don't honour the warranty by discrimination of the seller. It is a contract and it is not broken because the car is resold, official dealer or not.
RC:
2. The factory warranty is connected to various conditions and are you sure the current dealer really cared about them?
Except for outstanding service intervals and non-approved modifications, which both are down to the previous owner(s) and not the dealer, what are you referring to?
RC:
3. The official Porsche dealer can deny a warranty extension if the car hasn't been bought from an official dealer and/or if there are issues with the maintenance
Only true for issues with the maintenance, not where you bought the car from. I have already asked an official dealer about this case.
RC:
4. I'm pretty sure it is quite easy to buy a car from a Swiss dealer (as a french buyer), pay taxes and everything and then go back to the dealer when there are issues (used car = dealer who sold it actually needs to take care of the issues first).
Again, the warranty is on the car, not the dealer. If you buy the car from a private person, do you run back to that person to claim warranty? Just saying
Personal experience: I had an issue with a broken alternator on a 911 which was bought from a non-official dealer. I went by an official dealer, they checked what was wrong, they checked the warranty expiry date and replaced the alternator. No questions asked.
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Porsche 997 Carrera S in Carrara White with black leather interior. PASM-Sport Suspension (-20 mm), PSE.
Audi S5 cabrio in Ibis White with black leather interior.