Can't really tell what is concrete... But I think you might be far enough into the game for it to show up in a shop. Arthur sells a bunch of random things though, so chances of that particular item showing up in his shop is still pretty low.
I forget exactly when I shipped my own legendary scale, but I'm pretty positive that I did it before finishing act 3 because that took me a long while due to the difficulty. I got side tracked with other things like festivals and fishing/hunting for items to upgrade. It wasn't long until I saw it in Raven's shop right after, as with most other monster pieces I've shipped. If you haven't shipped a lot of the monster drops and gems, the legendary scale would have a higher chance of showing up in Raven's shop. At least that's how I think it works if you're looking for a specific item. You can probably try a few soft resets to see if it shows up, or try fishing up another one.