First, Black didn't sell the gun to Rittenhouse. He bought it with Rittenhouse's money and allowed Rittenhouse to use it.

Second, Black has been charged but not convicted, and I don't think the charge will stick: the argument is that while this was technically legal according to the letter of the law, the law in question was written with only underage hunters in mind, and should not apply in this case. Since the law itself makes no such distinctions, I don't see this as anything more than an empty gesture. Maybe I'm wrong.