Not a dealer here, but I can name several difficulties of starting a dealership business. Firstly, expensive startup expenses for newer dealerships. In other words, you will need a lot of money to buy all those cars, check them, repair them, and rent a building to place them all into.
Secondly, insufficient sales staff. In the beginning, you will not be able to hire enough salespeople to help you sell the cars, and you will have to hire workers with lower experience.
The pros of a used car dealership are that you will not have to spend that much money for each new car because they are all pre-owned. You will also not have to bother with much documentation. However, I still recommend hiring a financial advisor to help you not make big financial mistakes in the beginning.