If I remember the law right, you cannot buy a plant/materials and pay sales tax at that time and then mark the plant/materials up. Your State may be different but I'm 99.9% sure this is right for Texas. We have to charge tax for services also so we have to go to the State Comptroller and get a sales tax ID. With the ID you are tax exempt for buying plants/materials for resale. Then you take your final price for material and labor and add the sales tax on and then you pay the tax to the state.