Selling TBT isn't an option as OP is a new user and doesn't even know what it is. The best option for you would be to go to the island after 7 PM and catch the beetles on the palm trees and sharks in the water. What you want to do is remove all trees and bushes on the island, and leave 3 or so palm trees on each side, with a fair amount of space between them. After 7 PM, you'll be looking for the following beetles:
Golden Stag (worth 12,000 bells each)
Horned Hercules (also 12,000)
Cyclommatus Stag (8,000)
Horned Atlas (8,000)
Horned Elephant (8,000)
You could also catch the Goliath Beetle but it's only worth 6,000 bells. Personally, I don't bother catching them. Also keep an eye out for sharks, as they will sell for around the same amount as the beetles. I'd also recommend filling the perimeter of the beach with seashells to stop Hermit Crabs from spawning. Any bug I didn't list you don't want and should scare away. It should take around 25 minutes to fill up your basket with beetles and you can expect around 360k to 380k bells selling them at Re-Tail.
Your island should look similar to this one: