SO you might save the 10% mark up that is typical. And you may pay more for labor depending on where the material is stored. IF its in the way and has to be moved etc. If your material you buy is sub par the contractor may not use it, or charge you more in labor to make it work. My own take. Is the small amount you might save worth the risk of having the wrong material. Not having enough material. (one run to get material will be a hefty charge from the contractor.) or having too much that you have to return.  I had a customer decide to get his own sheet rock because of a good deal from a friend. He actually saved about 20% over my rockers price. However because of where he had them stack it it. It cost him about 15-20% from every sub for the problems working around it. (he refused to move it and wouldn't approve the cost of moving it. ) The biggest cost was time. His project went over a couple weeks and had way more in charges for a blocked work site than if he had just let us do our job.