As has been said, this issue varies from state to state due to sales tax.
The feds will be only be interested in the profit if you "resell" the item.
Since the original poster and a few responders are from Mass, I will only relate the facts as they pertain here.
If you line item your supplies on your invoices and add any kind of mark-up, you are a reseller and owe sales tax. All legal. It would behoove you to obtain a state tax number, not pay the sales tax at your point of purchase, and then add the 5% on the price you charge the customer. DO NOT TRY TO AVOID PAYING THE STATE THE TAXES YOU HAVE COLLECTED, you will buy yourself nothing but a heap of trouble.
If you do not want to bother with sales tax collection, invoice the job as a whole - do not separate labor and supplies AT ALL.
I did recently need to invoice with separate lines for labor and supplies (because of WC insurance). I listed the price I paid for EACH item, the tax paid at point of purchase, and then the total. That kind of detail is also legal. It proves I only passed on what I paid and indicated the state had already received its blood money.
Personally, I do not need to make money collecting taxes for any gov't. I am not in the retail business. My paper work is enough of a burden without collecting sales tax.