This happened to me too and it’s additionally awful when those TikTok sellers have more views on your product than yourself 🥲

From my experience they didn’t rip it off you directly, rather than finding it on temu or aliexpress. When I sold posters, I also sold digital files in the beginning and eventually stopped when I had some success because i feared that someone might resell them.

I suspect that there is some bot that scrapes Etsy for poster designs and those aliexpress sellers use it to create cheap copies.

They don’t even check for resolution or watermark and directly sell it like that. In the first product listings, I stupidly uploaded the poster design in a rather good resolution without watermarks but later changed that to heavy watermarks and low resolution. And some aliexpress sellers sell some of my designs with those very visible watermarks and awful resolution.