I run into this same thing alot with glass artists. Everyone uses the same techniques and tools but the important thing is to make it your own.
As for the bookmark in question, it sounds like the seller is "breaking the rules" but, I have to tell you, it happens all the time. Many people just don't care about the "morally wrong issue."
I always give credit where credit is due and I always email the originator of an "idea" for permission to use it for my stuff which, for the most part, is always a charitable thing. It's amazing how something as small as an email makes a big difference.
I would try to let go of the anger and annoyance over this because, chances are, you're going to run into this again and again and again. It's inevitable..................... You'll make yourself crazy.....................
It sucks, though, huh?