What do I do?

I have an important question.  Someone that I traded with has accused me of sending a fake card.  The card is Goblin Lackey from Urza's Saga.  I do not believe the card is fake, but I didn't test it before I sent it.  ( I honestly didn't even know there was a test you can do.)  He said that it fails the blacklight test.  Does anyone know what this means?  Anyway, I'm fairly new to the online trading thing, and I feel like maybe he's trying to scam me because he traded me something that went up in value after the trade was accepted.

My question is, what can I do to protect myself from another users accusations that I'm sending fake cards?  If the card is actually fake (which I doubt), it was by no means intentional and I would gladly send him a replacement card.  He suggested sending back all the cards that were traded to fix the issue, but I don't feel that's fair to me if the card is legitimate. Also, he only told me it was fake after I gave him positive feedback.

Any suggestions would be helpful. 

(I'm mostly just venting my frustration because I don't know what to do.)