What is the warning the pardoner offers with regard to greed?
Hello! You asked about the Pardoner in The Canterbury Tales by Chaucer. The pardoner's chief occupation is to sell pardons. People who want their sins forgiven pay for these pardons or indulgences; these are basically bulk certificates purportedly signed by the Pope for the forgiveness of sins. The Pardoner relishes selling his pardons and also his relics, which are supposedly items worn or owned by saints who were closest to Jesus on earth.
The Pardoner tells us that the theme of his Tale is radix malorum est cupiditas (Greed is the root of all evils), but is himself a loathsome caricature of a true pilgrim. Although the Pardoner recounts to the pilgrims the sad tale of the three rioters and the greed which kills them, he openly flaunts his own greed and his own avarice. He is a hypocrite and proud of it. By his great skill in preaching, he is able to warn his gullible listeners about the destructiveness of greed while profiting from their fears of going to hell.
That it is joy to see my busyness.
Of avarice and of such cursedness
Is all my preaching, for to make them free
To give their pence, and namely unto me.
He preaches against greed/avarice because that is what his listeners want to hear. It's only a business venture to him because he does not purpose to do any labor with his own hands. Through the story of the three rioters who indulge in drunkenness, gambling, gluttony and swearing, he warns that greed can only lead to treachery and death.
I include two links that might interest you below. Thanks for the question!