Just playing devil’s advocate here, is it possible they are selling a machine they made for their own use, and decided not to keep? Is it also possible they bought at least some of the parts from you?
Is it also possible they bought the kits from you, and are trying to sell it as assembled? That’s OK too, right? You don’t sell assembled kits.
If they are making their own kit, using your designs and selling it without all the proper knowledge to go with it, then 1) buyer beware, and 2) yep, get mad if it makes you mad.
No way they are going to get $2500 for it.