What is sales tax?

I am opening a gift shop whereby I am selling new merchandise. My question is this--when I purchased the products that I am selling I paid sales tax to the company that I purchased the products from so my understanding is that I now sell this merchandise to my customers as a reseller and I do not collect sales tax because I have already paid the tax. Is this correct ? I know that when I go to market and purchase items for resale I use my sales tax i.d. number so that I do not have to pay sales tax until I actually sell the items in my store.