In NJ the contractor pays tax on material at the supplier but the home owner pays sales tax on the material markup. So they say you should separate material cost and material markup on the invoice and only tax the markup. But then of course the customer sees your costs.

The law also says that if materials markup isn't broken down on the invoice the customer pays tax on all. So when I was contracting, the customer paid tax on all. I hated double collecting sales tax but I wasn't going to share my material costs.