There is a mountain of documentation of George Washington's actions and presence that go way, way beyond a merely rumored existence, including that his remains are interred below the rotunda of the Capital building in Washington, D.C. For the primer conspiracy theory to have even a fraction of the same provenance, you would need to have shipping manifests, freight documents, purchase records, and delivery logs for the supposedly waylayed primers (and not just Photoshopped versions delivered anonymously over the Internet, either). These sorts of paper trails exist in addition to ocean transport shipping manifests, bills of lading, and insurance records. So discovering how many primers have been imported to the U.S. in a given time period should be perfectly possible to do.
As far as sequestering primers to keep them off the market, mainly, hoarders are the ones who do that, even if it isn't their intent. I've heard economists on the radio describe how hoarding behavior keeps prices high for a time after a shortage ends. It is why toilet paper shelves were still pretty bare almost six months after production and supply lines were back to pre-pandemic normal. People just continued to buy more than they needed. Add to this that all the years of the push for gun control have made shooters suspicious and distrustful, and you have a pretty good recipe for both that sort of behavior as well as for adopting conspiracy theories.
Distributors are unlikely to want to get stuck paying property tax on large quantities of sequestered stock nor to risk the consequences of failing to pay their taxes, not to mention their accountants being on their backs about the opportunity costs of keeping the capital tied up. Fortunately, tax payment records are public, and if you can find records of someone paying taxes on hundreds of millions of primers, you might have an argument. But a far more likely industrial scenario is imported primers being siphoned off to ammunition manufacturers who are still scrambling to keep up with the new demand created over the last two years. The U.S. commercial market is something like 20 billion rounds of ammunition a year, and the reloading market is a pipsqueak by comparison to the loaded ammunition market. The money is in loaded ammo.
You probably don't believe George Washington existed since you can't see a photo of him.