Well it sounds like your best recourse is your credit card company. Best of luck with that and while this has been a painful lesson, I am sure you won't be buying a Mac "refurb" off of an E-Bay seller in the future. Going forward if you need to buy a refurbished Mac buy direct from Apple and if that is still not a viable option then buy from a reputable reseller such as Macsales.com.
If you are going to have the existing machine repaired, please take it to an Apple Authorized Service Provider, DO NOT take it to your local computer repair shop. Only use an Apple Authorized Service Provider so you know the work has been done right. If you do not know of any in your area, please click the following link to locate one:
Apple Authorized Service Provider Locator (AASP) & Genius Appointment
Best of luck in the future.