Think about it from his side, If he is telling the truth.
He ordered a used graphics card, seeing from the description that there is nothing described as being wrong with it, and then he gets it. It shows signs of being tampered with and doesn't work. He tells the seller about this and (I don't know all that you said) all he gets is that it worked for you earlier. He can't use it because it doesn't work, and doesn't want to just lose $150 to some guy, so he wants to get a refund but the seller won't do it.
I wouldn't be too happy either.