I think what you are wondering about is something similar to a prepared food, where the baker does not pay sales tax on the items included in the bread, but the end product containing the ingredients, the loaf of bread, is taxed. When the baker buys the ingredients, they are for resale, and hence not taxed.
I don't know the way it works in Virginia, as I am in Washington, 2000 miles further, but we do pay sales tax on oall our job related equipment here such as computers and software.
I do believe that contractors pay sales tax on their supplies here, but pass that cost on to the clients. They do pay sales tax on their equipment though. Professional service fees are not taxable in Washington though.
Mike McCann
MMC Engineering