I was wondering this too since I only just started out with NL recently and don't have much things to sell. I put the bell boom ordnance for my town and started catching bugs on the island, it's been helping me make bells much faster.
I got mine from making trips to the island, selling sharks and beetles. It was a pain but it's netted me my fortune through months and months of work doing it.
For me it was selling tons of turnips at my sister's town when they were worth 500 or so bells. Going bug hunting on the island also helps make a lot of money but is a very slow process so I recommend turnips. Also TTing to different seasons, getting event items and then selling em here for a lot can be a viable source of income tho a bit risky.
Selling or auctioning villagers and going into retail and selling an extra items laying in my town. Before I started with villagers I got a lot of money from just going into retail, looking at people who are buying things on their wish list, seeing if it's in my town or catalogue(order it if in catalogue) and making the exchange. People looking to buy things on their wish lists are very generous, and also giveaways. I've never cheated or any of that stuff but I am a cycler and that's how I make my money and get my dreamies. hope this helps!
When I first came here I thought that too, until Stitches moved out (for the first time), I auctioned him off and got over 10m! People really want there dreamies and to have a nice town so who can blame them?
I felt the same as you. I thought anything in the millions was insane. I actually try to stay away from any offers that would give me millions of bells. I like earning my own money in the game. It keeps it challenging/interesting for longer for me. If I ever end up with a village I want to get rid it, I would never ask for more than 500,000 for it, even if it's a "tier 1."
I got rich selling turnips, I kind of cheated really as I have two DS's so time traveled on one until I got a good price and then spent all Sunday morning buying and selling, I tell you, I never wanted to see another turnip after that day but it was worth it.