Replying to Ebay why did I sell anything there?

I fucked up and decided to sell a gopro on ebay since no one was buying it. I literally sent a brand new gopro 100% perfectly functioning the day I sent it out. Now I have some case opened against me because the buyer says I sent them a broken product and they want a refund, and I know for fact that the gopro turned on and functioned 30 minutes before I packaged it up.

I don't want them sending me back my camera that is broken. That is that. I am not sure what happened to the camera but I don't want to be taken advantage of by some buyer who accuses me 4 days after they receive the camera its broken and doesn't work.

Has anyone delt with this? The seller wants their money back but ebay said to not give it to them since I am getting a product returned in broken condition, which is far from the condition of the gopro when I sent the camera.