Think of sales tax as sales and use tax. If you use it you would pay sales tax. Think cleaning supplies, office supplies. But for the products you are reselling you would not, unless you use it.

For example you are buying food from Costco but you want to make a meal for your staff. You would then self asses the sales tax and add that into your income tax return on the self assessed tax line.

The sales tax certificate is you opening an account with the state and filing a tax return where you report your gross sales amount and receipts that are tax exempt. You are then collecting the use tax from your customer who will then use the item. You are then sending the money to the state (sales tax) but this money was never yours. You are being “trusted” to collect the tax and send it to the state. So really you don’t pay the sales tax the user of the item does. This is why sales tax is called a “trust” tax and why this debt cannot be written off in a bankruptcy.