How to sell popular products

Hi guys My idea was to try and sell products that are already selling thousands of units a month. Try find those with little competition also selling under that product. Check to see my margins are good, buy, undercut, i get the customers. Sounds great on paper. Now I have some more technical questions on getting this in place. By doing this, what are my limitations? I am just learning about the difficulty in selling branded products due to needing permission from the brand in order to do so. can someone explain more, and if this is the case, then which type of products am I limited to. For example lets say I find a smaller brand shampoo thats selling 4k units a month with only 2 other sellers. If I was able to source it for wholesale, then for a smaller brand would I be able to sell it under them? How would I know which I can sell and which I can’t. Appreciate someone filling me in on these right here and giving pointers