1st, Ebay isn't going to do anything. They try to give the perception of oversight and safety, which isn't there. If this were a sale with an individual, you are screwed. But it wasn't.
Call the Better Business Bureau and see what to do next. The Bait-and-switch routine is used all the time by dealerships, and is illegal. Then call the sales manager and give him/her a chance to fix it. It sounds like this price is above invoice so it isn't unreasonable to expect them to honor it. IF that doesn't work, try BMW NA and follow through with the BBB.