I feel like it's an unwritten rule that you Do Not sell merch of other people's small independent brands. It's just so hard to retain ownership of what you own online, and I've seen so many artists who essentially lost their illustrations to the meme sea when they made something that looked cool but then everyone else assumed that because everyone else was selling it without permission, it was fair use, and then were subsequentally dragged through hell.
Like all an indie person can do at that moment is ask in a blog post or something "pls don't sell my stuff thx" (like that's what Undertale had to do for a little spell there before they got superhuge) but like what more can a small artist do to prevent theft? Nothing, really. Like if this happened to me there's basically nothing I can do.
Like fanart is great and encouraged, but it only takes one repost out of context to completely lose the ownership of your work in the eyes of the world at large. Especially if it's a famous fanartist who's selling your stuff--they'll just think they made it. --Now if it's licensed fanart, that's another story, but I'm gonna assume everyone selling in an artist alley did not get that.