What do they sue you for? If you are upfront about what you are selling, and you didn't steal any IP, it isn't fraud or theft of trade secrets. Maybe they have

What do they sue you for? If you are upfront about what you are selling, and you didn't steal any IP, it isn't fraud or theft of trade secrets. Maybe they have patents, and claim you are violating them? I can't see how it is illegal to reverse engineer a part and sell a similar version.