A blacksmit villager let me to sell him 9 iron ingots for an emerald, even when his trade bar showed 20 coal for an emerald. This 9 iron ingot trade was real, it was the 3rd trading option, just I could do it in the 1st trade option.

So the Bllacksmith should give me 1 emerald for 20 coal.

It also let me to give him 9 iron for 1 emerald, on the same trading bar as described above.

Steps to Reproduce:
1. Create or find a blacksmith villager.
2. get it to its level 2 trades.
3. check how many iron is he needs to give you an emerald.
4. put that many iron into his 1st trading bar (where you could sell him coal) and you should see, that he is willing to give you the emerald for the iron.

Its not a realy big deal, but I think it is not how it should be.