Trigger event on successful trade with a villager

In the same way that it is possible to Event Trigger when a "Player fishes item" how could I Event Trigger when a "Player successfully trades with a villager"?

I want to trigger an event when a player successfully trades with a villager.

I've asked loads of people and no-one can help me, i'm open to the idea of custom code, I tried to do it but could not figure it out.

Even if this is not possible, how can I read a Villagers XP tag as this would allow me to figure out if its increased since you last right clicked the Villager and trigger it this way.

Any help is appreciated :)