We're AAA members and I found a couple of posts on this because I was wondering the same thing. Many AAA offices only offer the parking passes if you book your trip through them. However, a couple of people have said they have been able to get the passes from their local office without booking a trip. I'm going to stop by our local office to try my luck, we didn't book our vacation through them because we could get better deals on our own.
I would be cautious about the Ebay option, one post mentioned that the passes now include name/dates on them.
The Diamond passes are only supposed to be available through AAA. The ones being sold on Ebay are not legitimate. It clearly states on the pass that they are non-transferrable. And as the PP said, I believe they are trying to cut down on the fraud by putting the actual AAA members names on the passes.