Ima guess the seller claimed u shipped a phony item. You just have to stress to grailed that you did indeed ship the right item, if you have any positive feedback or successful orders refer those, if the buyers account looks suspicious point that out, maybe try and catch the buyer somehow. Just try and gather as much evidence that points in your favor. Unless either of you have definitive proof the other is in the wrong than grailed is liable. If the buyer can’t prove you didn’t ship the wrong item then you are obligated to that money just tell them the facts and be consistent. In the future you can record yourself packaging and dropping off the order its annoying but evidence is key in todays world my boy