You can be a tax-exempt organization, which means you do not have to pay sales tax on the items you buy. Examples of this are government entities, certain non-profits (some are required to pay), most churches, schools, etc.
Or you can be a reseller. For example, if you are purchasing printed banners from a wholesaler, you do not need to pay sales tax, since you will be collecting sales tax from your client.
In both of these cases, there should be a state tax exempt form that you should require from your customer before offering them goods tax free. If I don't have the paperwork, I ALWAYS include tax, even if I know they are tax-exempt. Seeing it on a bill often gets them to bring me the form. I have a LOT of exempt clients, and I worry about getting audited. This way, I know I won't be screwed in the long run.
Hope this helps.