1. Perfect fruit. Do not sell your first perfect fruit, use it to grow more trees. Get 20 trees goin and make it rain!
2. The Island. Head there about 5 pm cause that's when beetles come the most and sharks aren't too rare. (11PM sharks are everywhere, but is kinda lame that no stores are open at that time). Scare off any small fish/bugs. Use the net if you want to scare fish, but running past them works fine. Catch locusts and release. Pretend to try to catch a tiger beetle/wharf roach by just using your net on empty ground. If you get a sea bass, you can just release him out to the water where a pesky small fish is (SEA BASS TORPEDOO!!! XD). Rinse and repeat, and I get a full inv of big beetles and sharks in under 20 mins. Usually sells for 120K or more. Most I have gotten from a full inv was 174,000. This also leads into #3-
3. The Campsite. After visiting the island, and if there is a camper there, take a half full inv and lots of bells. Occasionally they will want to play a game with you for one of your items. IF you got a big beetle/shark, they could buy it for a ridiculous amount if you are lucky(1 Elephant Beetle sold for 43K, i sold a shark for 200K). Biggest loot that I can find. Try your luck there, but I warn you that you CAN and likely WILL be swindled from time to time but the loot is well worth it.