As far as I know from dealing with collegiate licensing, you only need it if you are the manufacturer of the product. For example, if you are a screen printer and want to use a NFL, NBA, etc. logo and/or likeness, you have to obtain a license to do so. However, to resale something such as a NFL jersey, Reebok (the manufacturer) has already obtained the license and you are just a reseller. I would mainly insure that you only buy Officially Licensed merchandise in order to protect yourself. In other words, don't buy a bunch of non-Reebok NFL jerseys from Hong Kong.....those aren't official.
Bingo, the manufacturer has already paid the copyright holder a fee to manufacturer under licence.
You are permitted to sell the product, just so long as you are sourcing the product legally and selling a genuine licenced product, and not a fake!