Adding Effect if player has a spezial item

i made a opponend, and if you kill the opponend you get an item. So, and until here is everithig clear, but how can i make that the prices of villagers lower if you have the item, so that they are insanely low? I thought abbout hero of the village, but i didn't find out how to apply an effect if the player has a speciffic item in his inventory.

THX for help

Btw sry for grammar and other mistakes