Where you enter the materials depends on how you sell them. If you are using the supplies as part of a repair they would be entered in the expenses section as supplies. If you are installing a new bathroom, for instance, you may count the fixtures as inventory.
Another consideration is how you purchase the materials. Do you have a inventory of sinks, faucets, and other fixtures on hand that the customer can select? If so this is more like inventory. If you purchase the items as needed for a job then they could still be inventory, but the are more likely supplies. If you do not have any materials left over at the end of the year the effect on your tax return will be the same if you call them inventory or supplies.
Any small parts (like screws, o-rings, sealant, caulk, etc.) used should be considered supplies and entered as an expense.
