This seems relatively simple to me. The original poster (seller) took a cashier's check as payment. The OP of this thread (buyer) alleges that the items he paid for were not in the package (an envelope) he received. The Seller swears he sent everything, but was concerned about whether the cashier's check was good. The buyer swears the check is good. So, here's where that leaves us: The buyer needs proof (always hard to prove a negative) that he didn't get what he paid for. As for the seller, the check is either good or it isn't. He's been encouraged to call the issuing bank to confirm its validity. If he's done that, he hasn't shared that info here. The fact that he got the money doesn't mean the check isn't counterfeit. It can take a while for that to be determined.
It's a mess, but it isn't all that complicated, factually. The buyer has done exactly what we should expect - given his side of the story. So has the seller. One of them is lying. We have no way to know which.