This comment brings up the reason why I'm finding this one difficult to judge this OP - so one person copies a picture of a picture that another person has done of a kangaroo, and you make a picture of a kangaroo from a real life kangaroo. But it's a kangaroo, and there's really only so many images of it you can make until huge numbers of them start to look similar, or the same.
People have been making images of kangaroos as long as there have been people and kangaroos, that's millions, or even billions, is images. It's hugely unlikely, to outright impossible, for one person to make one now that is so unique that it doesn't look like any other image of a kangaroo that's ever been made before. It doesn't matter how much more time you spent on it, or how you went to look at an actual kangaroo, if it looks almost exactly the same as thousands of other kangaroo representations that are already in existence.
I don't disagree that copying your work exactly, then emailing you and telling you that they've copied it and they're going to sell it is probably a breach of copyright law - they've literally told you they're doing it. But I'm struggling to believe that your work is so unique that you'd be able to sue for copyright infringement just because you see something that looks really similar to yours. You also said you've done online, free tutorials on how to create your crafts, which could be used as an argument against breach of copyright because you were clearly ok to share your ideas then with the public, meaning that if they then choose to create something that you've created before it could be down to the technique that makes them look the same, not because they've seen it and copied it.
Basically it's just hard to tell without seeing what you do, and how unique it actually is, but I think it's unlikely it's so unique that you'd reasonably be able to sue without the person actually stating to you that they used it for the purposes of making money. And I think it's unreasonable to get annoyed at people making their own copies for their personal use, that falls under fair use. It's also a great ad for your stuff, as their friends will ask them where they got it, they'll say they made it but they copied it from your website, so the less crafty person may go to your website and buy it. That's a win, really!