Ask HN: My Open-Source project is on eBay. What should I do?

I just find out that my GPL'ed CNC software, CAMotics is being sold on EBay. The seller clearly made modifications and I don't see any way to access the modified source. Isn't this a license violation? Has anyone else had this happen to them?

https://www.ebay.com/itm/CNC-simulator-software-for-CAD-CAm-milling-machine-or-router/113808167909

Real software: https://camotics.org/