I used to work at AAA in NY. As others have said, it depends on your own clubs rules regarding the parking pass. In my club, they would only give you a parking pass if you purchased a Disney package through them (even though they had a drawer full of parking passes but I guess that's their leverage), in the downstate NY club, they would give it to you if you were just a AAA member - no purchase necessary (other than the membership). I do believe for most clubs you would just need a basic membership and the benefits would be the same. However if you're getting an AAA membership for the road service benefits as well, you should look into the higher levels of memberships because some of the benefits are really worth it. Namely the towing, in most clubs.
With regard to the parking passes on eBay, it's not allowed to sell them and if you had a parking pass and no AAA membership and card to go along with it, you could be asked to leave. I don't think anyone asks for your AAA card in the parking lot but they could. In some clubs, they change the design of the diamond pass every year so it's possible the ones on eBay could be old designs too which would be a giveaway.