I've always put high-valued fish and bugs in my storage to sell to CJ and Flick, respectively, as they pay out 1.5x the amount of bells Nook's would offer, even with bell boom ordinance. The bells will roll in quick once you save enough fish and bugs.
There's also a very cool method I discovered involving a specific DIY, but it's complicated. Basically:
- Bell boom needs to already be in effect. Telling Isabelle to enforce it the same day won't work, as you need to wait a day.
- You need at least 297,000 bells in your ABD. Crafting the item requires 3x 99,000 bell bags.
- You need the "Pile of Cash" DIY recipe, and the hot item at Nook's has to be this, so check the sign often if you have the recipe. You can obtain it from Snooty villagers or message bottles. As this is a 2.0 item, the chances of finding it are higher if you look around the HHP Archipelago beach or go on Kapp'n tours.
- Have plenty of pocket space. You can carry up to 4,059,999 bells with 40 spots. I would highly recommend only selling 6 or 7 Pile of Cash items at a time with nothing else in your pockets.
Normally, selling the Pile of Cash item directly to Timmy or Tommy just gives you your bells back if you have bell boom, and you'll actually lose bells if this ordinance isn't in effect. If it's the hot item with bell boom, you'll have the opportunity to farm bells infinitely until the store closes. The best part? You don't need to be online to do it. When this happened to me, I made 54 million bells easily, and was already set for Animal Crossing retirement.
It's indeed a very difficult process, as the chances are slim to have that DIY be the hot item, let alone if you already have most of the DIY recipes as they randomly cycle alongside the food ones. You just gotta get lucky.