Usually when you open a PayPal dispute with the reason of a counterfeited item, PayPal will require to you to verify the item is counterfeited by getting a written letter from someone that works at the store, and in this case, a letter from an employee at MAC.
What I would do is have a pre-typed letter that includes the reasons why it's fake compared to the original one (that's only if you already compared your fake pigment/eyeshadow) along with the contact name and number that works at MAC in case PayPal needs to contact her/him to verify. The best person at MAC to sign the letter is the manager, IMO. Make sure to explain in details to the manager on what's going on about the whole situation then have her or him sign the letter. The reason why I suggest having a pre-typed letter is to save the manager some time since most likely you know what to pinpoint that's fake by using and reading guides here on Specktra or other websites, etc. After that, you can fax that to PayPal or email a copy of that to PayPal, and you'll win the case and get your refund. On the letter, make sure you reference your eBay user ID as well as the item # and the seller's ID also and the MAC employee name, contact number, as well as the location of the MAC store.