It's faster to sell TBT for in game bells on here.
However, for in game methods, selling bugs and fish are my personal method, as well as gyroids, fossils, extra furniture. Anything like that.
I like to earn bells in a variety a ways.
Most often I go beetle/shark hunting. First off the island is one of my favorite places to go in AC:NL, but when I need money, I turn to the island for help. Every trip I make gets me around 300,000 bells since I only catch bugs that are worth more than 8,000 bells and any sharks I see. Even though it takes a while to fill my box with the desired creatures, it lets me train myself for catching bugs/fish in tours, and the bug-catching event/ fishing event. Not only that but they help me fill my encycloedia to get the badge as well as my general bug catching/fishing badge. And when I want to take a break, there are tours. I can even gather flowers on the trip! Anyway, the island is my best option for making bells when I don't have turnips with me. My technique is to cut down the trees in the grassy area so I can only receive palm tree beetles. I also dig up stumps and discard flowers so I wouldn't get any mantis or butterflies. I start hunting around 5PM even though, they are common at 7PM. On Sundays, I buy turnips to sell to Reese. I use my 3rd character to collect turnips by having her fill her entire locker with turnips. Then I sell the turnips at somebody else's town usually for about 500 bells or more. Lastly, I try to join any giveaways I can find. Just winning bells out of the blue is amazing. I once brought home 20 million bells from a giveaway.
I fish, catch bugs, and clean out my closet of anything I don't currently want and can order again. I've never even tried the stalk market, I only have a vague idea of how it works and I'm too impatient. I've thought about trading on here? I've only ever done trades thru Reddit and it wasn't anything big. I don't even know what kind of stuff people pay bells for that I would have.
I'm a turnip professional right here. I remember That one time I found a good price in my sister ds which is 518. I end up doing 3 weeks of turnip non stop. XD
Then you can trade your TBT for IGB in here. I like to use TBT for villager trading instead of IGB. It's easier and faster. I don't have to clean my inventory when i need to trade XD
In my third town (Croydon) I've been going to the island a few times a week. By having the Bell Boom ordinance in effect, going to the island at night, and filling all 40 spots in the island's take home box with the rare and pricey bugs and fish, I usually make about 350,000 to 450,000 Bells for each trip.
Bugs and perfect peach farming. tbh i only tt when im dealing with moving villagers, i prefer to wait until night to go bug hunting. however, i always end up falling asleep so in the past few days i havent gotten my usual haul.