I understand the frustration. I've seen re-pops of my stuff showing up too. I was initially pretty mad, but just came to terms with it. Resin casters come and go, most of the people who buy from people who sell re-pops are going to get ripped off sooner or later. A lot of these businesses fold when it stops being fun. If you take a risk at buying a knock-off from someone to save a few bucks, you have to accept the consequences when the stream of excuses as to why your order hasn't shipped for six months starts coming.
People that are in the business for the long haul work hard to build a reputation as a solid and trusted entity. We give up our nights and weekends to make sure that orders are filled in a timely manner, and we don't sell what we don't have in stock. I would be leery of anyone who sells cast parts to order.
Legal proceedings against a re-pop caster is a waste of time and money.
Just ride on your reputation of trust and quality and time will sort out the rest.
I wouldn't even post about the person re-popping your stuff.. it just drives traffic to them, and it's free advertising for them.