Full dealer service doesn't really mean much to me. If people love and look after their cars, then they generally keep all the paperwork. If I wandered up to someone for a sale and they handed over a nice wad of paperwork that showed everything was done that needed doing. I wouldn't give a monkeys if the service book had Audi stamps or not.
When I sold my last car, I gave the guy an A4 folder full of everything from full exhaust and suspension replacement invoices, right down to invoices for replacement wiper blades and bulbs. He couldn't believe it. He sat there for 5 minutes looking through everything. I think it showed him that I looked after and loved the car. It only had one dealer stamp. The rest were all from a specialist. One known within that community. He had done his research, so he had heard of them.
Some people get hung up on dealer stamps, but that isn't always necessarily a sign of quality or assurance. It may help with resale for some but it wouldn't put me off if you could show the car had been looked after.
My A3 had dealer stamps up until when I bought it. In fact, some were missing, so I contacted the Audi dealers to get the missing stamps filled in. That was more about me being able to show regular servicing than showing Audi had specifically done it and I wanted the stamps in the book to match the invoices I'd been given by the previous owner.
I now take it to AmD as they are well known, their work is good and their prices are much more competitive than the main dealer.