My main business is metal fabrication, mainly for off-road vehicles. Jeeps and such. In this marketplace it's extremely common for ideas to get ripped off, copied and sold for less. It's just how it works, and too often how the world works. If I got upset everytime someone stole one of my ideas and started making money from it I'd have no time to build anything new. It's not like you have a patent or a trademark on these things. Yes, you spent time on them and I understand your frustration, but the cold hard fact is that you're making something that has been made a thousand times before, and that you cannot control the duplication of.
Is the guy a douche? Fuck yeah.
Can you do anything about it? Fuck no.
Time to put on your big boy pants and be happy with the knowledge that the products you build are of a higher quality than theirs and make sure that you promote your brand accordingly. People that care WILL buy from you. Cheapskates that dont, probably wouldn't have bought from you to begin with. They would have gone to one of a hundred other ebay sellers.