OP remember the old saying you can catch more flies with honey than vinegar?? Well you had a great bargaining chip in your hand, (which has now likely been lost).
That was to be polite and courteous to the dealership and ask to speak with the GM. then when you had his attention ask for an in store credit for the amount of your deposit. Then state you are member here and you wouldn't want to see the dealership get any bad press over this unfortunate turn of events.
Now having said that you, (full credit for doing so), have admitted the dealership has really done nothing wrong in this transaction. They took the deposit in good faith and you at the time in good faith signed a contract which stated the deposit was non refundable.
The ideal outcome is the dealership works with you on this.
Now that you have named the dealership the GM has two options.
1. give your deposit back or a credit for use in store with them, (which you will no doubt post and "some" will be happy to say they are a good place to deal with.
2. Tell you he is not interested in your situation and as the contract states the deposit is the dealerships to keep and that is what they are going to do.
If they choose option 1. there will be some who say good for them but I am never going to buy from them regardless.
If they choose option 2 then there will be many who will say they are crappy and they will never shop there, (mainly those who have already said so in this thread).
Reality is he is not going to win those people back by giving your deposit back. This thread will go on for a few more days and then GTAM will move on to other shiny things to look at..lol There will be "some" who may use the dealerships name in the search feature and find the thread at a later date. But for the most part it will be gone and forgotten. By you having already named the dealership the damage is DONE, it can't be undone even if they return the deposit. So he can say well I have nothing to lose by keeping it at this point and $500 to gain. I ma not going to regain the loss of reputation by returning it.
I think you would have faired better had you followed your initial instinct and not named the dealership, UNTIL you were sure you were out the $500. But even then by your own admission they did nothing wrong.