Players can isolate a villager and place a workstation near it to have it become a merchant.

If they do not like the trades the villager acquires, they can simply break the work station and wait for the merchant to become un-employed.     Upon placing the workstation again the villager will receive a NEW set of trades (even with the same profession).   

This allows players to easily cycle through trades until they find one that is more desirable.  A video is provided here of a user exploiting this on a large scale to collect a group of villagers with perfect trades.

https://youtu.be/hLNqOTi9PHs