I wanted to test out merchant in a single player world before I chose it on a server. Whenever I trade more than the max vanilla amount the game freezes, mobs stop moving and the game gets stuck on 'saving world' when trying to exit.

Were you by chance playing as an Arachnid, and on Origins 1.0.0? There was a bug with Arachnids where the world froze.

No, I was on the most recent version. I was originally avian (as that’s what I play as), but I tried it as a human as well and it still crashed.