What they seem to be referring to is licenced products, not that every product is licenced. For example, an iPhone isn't, but a Disney doll may well be. Products are licenced to protect against pirated fakes, so you will require the correct authorisation for licenced products.
Now having said that, you wouldn't be dealing directly with the manufacturers or licence holders anyway, not unless you were a serious wholesale customer. Your supplier will be at least one link down in the chain, maybe wholesale or even further removed, a dropshipper. What the licence holder will do is authorise the wholesaler or supplier of type, with the authorisation to distribute their products. So once you purchase from them, then it isn't an issue.
Of course the issue comes when people think they have found a killer price on the same item, then end up purchasing a lot of fakes. Of course you are asked to provide certification, which any legit authorised supplier will be able to provide on request, but a fake maker won't. So end result, shop is closed down ... and that's the best case scenario, because depending on the scale, there is always a chance of legal prosecution that could follow.
So always use legit suppliers and if in any doubt at all, always ask for authorisation documentation. Provide that up front to them, even to someone like PayPal to avoid any red flags there as well and you can get off to a clean, hassle free start.