Wow, I'm infuriated for you. I would definitely look into taking some sort of legal action. Contract formation is based on an objectively reasonable standard and, based on the facts you've described, you (in my opinion) had every reason to believe that the dealership would honor its offer price (no reserve). Check into the terms and conditions of ebay first, though, to see whether or not there are any alternative dispute resolution methods that you must exhaust or go through. Good luck.
2007 E92 335i | Space Grey | OEM Aero Lip | OEM Aero Bumper | M3 Spoiler | M-Tech Sideskirts | Painted Reflectors