If it's sold in the US for retail consumption and it's never been taxed, the tax is due. The tax is collected at purchase. I don't know how items could be "diverted". Nevertheless, cheating is going to be a problem in any tax system. The intent is to minimize criminal activity. You're never gonna eliminate it. If the tax is fair, easy to understand, and it's easy to get caught with significant penalty then cheating would be lessened IMO.
"If it's sold in the US for retail consumption and it's never been taxed, the tax is due. The tax is collected at purchase. I don't know how items could be "diverted"."
Anything can be diverted. How do you know if the tax has been paid without documentation, ie, a tax stamp?