Will I save money if I buy the materials, instead of the contractor buying them?

He reroofed part of the house and there are 2 bags of shingles left over and they are opened. He also bought twice as much insulation that can't be used...We have a lot of work to do, but if there is going to be that much overbuying and throwing it away, would I be better off to change my contract to labor only with us furnishing the materials????