I think the correct way to contact them in this case is to have your lawyer send them a nastygram.

Just because you only have one authorized distributor does not in any way mean that stock being sold by another party is counterfeit. After your product is sold there's nothing preventing the buyer from reselling it themselves. Clearly a lot of products are most likely counterfeit but assuming they are with no proof is the same draconian logic used by all of those copyright protection firms that send out automated DMCA takedowns without a human ever reviewing the takedown first.

That would be the case if they were actually reselling our items, but they're all taking our logo and putting it on print-on-demand t-shirts, hats, mousepads etc. I've not found a single listing that's even remotely close to a resold product.