What does the Pardoner mean by "Covetousness is the root and stuff of all I preach"

The pardoner is defined by his greed, his trickery is getting people to give him money and goods. He covets wealth, and he acts as a pardoner in order to get those things. However, the irony is that he preaches against vices like covetousness, encouraging folks to give up their money in order to be forgiven for their sins. So the sin of coveting is the root - it encourages him to preach, the reason he does it - and also the stuff, since it's what he preaches ABOUT in order to get that which he covets.