These are the functions which can be called on a minecraft:villager_trade criteria 
trigger.



getEntityData:
    Notes:
        Returns the EntityData object so functions can be called on it.

setItem:
    Arguments:
        ItemBlockData
    Usage:
        item data
    Notes:
        Sets the required item that must be traded for from the villager.

setEntityType:
    Arguments:
        String
    Usage:
        entity type
    Notes:
        Sets the entity type.

setCount:
    Arguments:
        int
    Usage:
        count
    Notes:
        Sets the number of times this criteria must be completed before it counts towards advancement progress.

getPlayerData:
    Notes:
        Gets the PlayerData for the criteria. Can set many things to test against the player themselves.