Let's Go Early Game Money Grinding

Money is a necessity in Pokémon Let's Go: Pikachu! and Pokémon Let's Go: Eevee! You can't buy Pokéballs without them.

Fortunately, the developers realized this and implemented an emergency NPC. Inside of Mt. Moon, there's a man who will give you 10 Pokéballs if you're out (or have below 10 - need to verify this one). This is unlimited.

What you can do here is sell all your Pokéballs, and get 10 from him. Use one Pokéball, then talk to him for another 10, bringing your total to 19.

Sell your 19 Pokéballs for that sweet, sweet money, and repeat.

Unfortunately I'm guessing the efficiency of doing this tapers off as you progress, but it's a nice feature.