One of things I've discovered is that the Flea Market in Re-Tail is an excellent way to make quick bells. When you put an item up for sale as part of the Flea Market, there is a default suggested price that is displayed, which is typically what Reese will pay you for the item. I have found that the villagers will generally pay you double the suggested price for general items like clothing, furniture, flowers, etc. which is great, however, believe it or not, the villagers will pay up to 9,999 bells for fossils and gems and ores that you get out of the rocks like silver, gold, rubies, amethysts, emeralds, sapphires, etc. even if
the suggested retail price is substantially less than that. From what I have found, the villagers typically will not pay more than 9,999 bells for an item, so if you price an item at 10,000 bells, they won't buy it. Also the animals don't seem to want to pay the 9,999 bells for fossils like the fern fossil, ammonites, amber, etc., but you can probably get about 5,000 bells for those items. I have also found that you can sell most gyroids for about 5,000 bells each. I recently even sold a beehive for 2,500 bells, which definitely helps take the "sting" out of the bee sting I got when it fell out of a tree!
Sometimes the villagers will say that the price is too high, but eventually they will come around and spend the bells. I have made quite a few bells selling to the villagers using the Flea Market in Retail, and that really helps out when a public works projects are costly, and a single room expansion is almost 500,000 bells!
Of course, if Reese is paying extra for an item during the day (you can find out what items those are by looking at the board in the front of Re-Tail), she will pay you double the suggested retail price for the selected special item, and if you have a Wealthy (Bell Boom) Ordinance, she will pay you 20% more for everything you sell to her.
I'm sure there are other ways to score buckets of bells, so try out various things and go get your bells!