I can only speak for myself here - but today just experienced the very same thing the OP has mentioned. I was not happy to see my work of 4.5 months so easily plagiarised and being put up for Auction. It especially incensed me because I myself, had not even put this line up for auctions and had only sold 6 kitties.
Luckily the auctioneer agreed with my point of view, and the "breeder" in question swapped to another kitty.
When I purchase a cat - its with the sole intention of improving my cattery, taking the traits needed and working them into a favourite line. Not with the intention of copying the exact kitty for marketing purposes.
But as already mentioned so eloquently by several posts already - we cannot police these situations - and yes - once you sell a cat - its fair game what happens with them afterwards.
Oselkhandro your post has made me seriously rethink my pricing thank you.