I don't have advice, but why do you not want to name names? They are clearly in the wrong. Links and information will get you useful advice about how to proceed.

You're not the bad guy. Sounds like you've tried to clear this up privately, and continuing to protect this person helps no one but them.

This is wise of you, actually, especially if you want more legal leverage.

DCMA every place where they're sold, his hosting provider, and make sure you collect any information about the person so you can later go to court, if needed.

Where are they selling them exactly?

If it's not on their own website- something like Etsy for example- and you have solid proof that the box is yours and what they are selling is merely reproductions, you may be able to report the item and get it taken down with that proof.

If its on their own website, or on a website where non-handmade things can be sold (like ebay) then you will have a tougher time with this. You can try a DMCA but that is usually for intellectual property rights rather than actual production/sale rights for physical goods. Besides, its not that they aren't allowed to sell them, but that they are claiming that they created it when they didn't and thus denying the original creator of the credit they deserve.

It's a tricky situation in the sense that, while the "wrong"ness is clear cut and easily visible, it might be hard to resolve, considerably moreso than most standard art theft on the internet.

I'm going to do some research and see if I can figure out where they're hosted at, and give their ISP a call.

While you can't control the fact that this second party is selling the boxes they bought directly from you, nor how much they're asking for them, you do have the right to remove their false copyright. The copyright for the design falls to you and you alone. You can DMCA the website that the seller is using, but that's likely about the limit of what you can do.

It's not illegal to resell products, nor is it illegal to markup the prices that you sell them at. They just can't claim that it's their own design, is all.

I don't care if they sell them or not - they bought the originals and as far as I care they can smash them all with a hammer. It's the claim of copyright that's the issue here, as stated in the OP.

i don't understand that

oh, yeh ok.

I assume you have your transaction info still? E-mails or notes or Paypal logs?
Maybe photos/etc. that predate anything of theirs?
Not to mention a number of similar products or other things that indicate stylistic similarities?
There's your proof.

I don't have any helpful advice but that sucks, so goodluck getting them to stop!

If they're on DA, DMCA DA.
If they're on Etsy, DMCA Etsy.
If they have their own homepage, DMCA their host.

I just wanted to drop a note for everyone here's information. This type of activity (claiming copyright of something which you down own the copyright of) is called slander of title in the legal world. It might not be quit straightforward to use a DMCA request to get rid of this, but you can write out a cease and desist letter and send it to their host's DMCA contact and see where that goes.