Have you tried it on a different player? Maybe it is just the one you are using. Not a help, I know, but I have had things work one place and not another, meaning maybe your player could use a cleaning. Try your computer if it has a dvd drive.
As far as eBay goes, I have had nearly problem free experiences both buying and selling. As a seller, I would suck it up and refund the money. Regardless of anything. If I knew in my heart that an item was perfect, and someone said it was flawed, I would have them send it back and when it is returned to me, I would send them back their money. One time I sold a pair of mint Dansko shoes. The photo was clear, and the shoes were in excellent condition. I described them as used simply because they were from my closet and not a store. I received my one and only neutral feedback, and was really annoyed. The buyer never even once emailed me about it. She said the shoes were mediocre and badly described. I would have happily taken them back and done whatever it took, but was never given the chance. Never even heard from her.
I know the seller you are dealing with is being a bit testy, but I would still ask her to take it back and refund your money, especially if the disc doesn't work on a different player. Good luck!