I usually grind bells on the island at night. Occasionally I will exchange my TBT for IGB on here. In my second game I have a lot of bells because I sold my old town
My main ways have been bell boom together with bug/fish farming my own island.
Occasionally I've made bigger amounts by selling perfect fruit (that is foreign to my town) when its been on premium & turnips if I get a high price.
The latter two needs either the ability to wifi with another persons town or more than one town yourself. Also neither of these are helped by being on bell boom (ie bell boom does not increase the amount you get for fruit on premium nor turnip prices).
I've made 50 million for the mayor this way in two towns, - I aim for that as I like the mailman's hat you get at that point.
Sometimes I sell TBT here for in-game bells, but generally, I go to the Island to look for bugs. I'll only catch bugs that are the value of a Goliath Beetle or higher, though; it makes it quicker than if I catch every beetle I see.
I also harvest and sell all my perfect fruit in other towns where it isn't the local fruit.
On the off-chance I'm lucky enough to have a Lazy villager camping in my town, I'll play games with them, because they'll buy my items for a higher price than Re-Tail will.
I just bug grind. I mean it's not like it gets you a TON of money but I get a good 200k each trip, sometimes a little more or less, but I end up spend it all the next day on PWP's and house upgrades so I don't really "save" a lot.
Trading on the forum is how I earned the most bells. TBT to igb is fastest.
I used to have villager adoption sales. Before the amiibo update, last year it was rather easy to make bells. Someone wanted Beau really badly and I became a bellionaire, lol.
Sometimes I also like selling perfect fruit at other towns.
The fastest way without exchanging tbt would be to buy 1 million bells worth of turnips and sell them to Reese when she's paying a lot for them. You can easily make 2-5 million bells with this trick on one trip alone
I take trips at night to the island to catch the most expensive bugs. I only catch the ones that are worth 8k or more, and just fill up the whole box. I usually make around 3-400k each trip. It's my way to unwind at the end of the day since the island is so relaxing Otherwise, I made the most money by selling my old town. The tree has to be level 4 (so it will show 4 branches) and then you can sell. In my last town I made 16m which isn't a lot compared to most on here but I barely had anything in to it!
I sell perfect fruit at premium in my other towns. A locker, pockets and letters full of perfect fruit sold at premium will get you 11 million bells.
I do the buy/sell turnips on the same day when one of my towns has a high price. I have a spare town that can change dates to Sunday. Made 80 million bells.
Bug hunting during the summer can make you a lot of bells. Bugs can be premium at retail. A goliath beetle at double will get you 12K. Golden Stag 24K.
When there's a lazy camper in town I sell them all of my hoarded 'high' value items. hasn't been a lazy camper in town for a long while though, unfortunately.
Normally spending all night at the island lol. Getting a whole box of Golden Stags would get you rich quick, but I haven't been lucky enough to get rich yet. I tend to forget about turnips for the week, or spoil them by TTing without realising I have them, so I always lose bells instead of earn any with turnips.
There's grinding for beetles, sharks on the island...
Some people sell villagers...
Some people sell items...
Some people play the stalk market...
The fastest way is probably to sell your town to Tom Nook (but I know everyone doesn't want to go that route and lose their favorite villagers - some of us don't have amiibo cards/scanners T_T)