I had two villagers, a master farmer and a novice librarian. i had traded with both, so their trades were locked.

they got turned into zombie villagers, so i cured them. when they came back they still had the same trades and levels, but all their trades except the potato/carrot novice trades from the farmer cost exactly 1 emerald or 1 of the item they give an emerald for.

So, I have bookshelves for 1 emerald, and i can sell books for 1 emerald. infinite emeralds. Mending book is 1 emerald and a book. I can sell 1 melon for 1 emerald. All of their trades except carrot/potato are like this.

I tried relogging, to see if it was a visual bug, it was not. My friend came and looked at the trades and they were normal values for him, so I'm the only one with access to these trades.