If he has no priors, I can think of one thing that could possibly have happened. :-\
For example, I have some MIB YYF yo-yos sitting around that I got from other sources. Mine happened to come from stores. If I sold them, I would not inspect them first, because of the source.
But, if he got that Superstar from an individual who traded the yo-yo to him as MIB, and the box looked mint when he got it (as it looked new to you), he may not have ever opened the box to inspect the yo-yo. Most people wouldn’t open a new yo-yo to inspect it, so maybe he made a presumption about the newness. People who trade a lot might get a traded yo-yo, and not open it because if they open it, they might damage the packaging. If he got it in a trade, just intending to trade it again, he may not have seen the need to inspect it. I think he should have inspected it, because it’s a trade.
So, you should PM him, and see what his reasoning is about how it could have happened, and see if he will correct it in a timely fashion, in some way that is agreeable to you.