Here's what I've been doing to make silver and gold beetles without spending currency. I'll be revising and adding to this over time.
TC = Trading Center

Selling Food
I can consistently sell batches of food on the trading center. I sell food that I don't need to keep my lions from running. Try to use food that isn't expiring soon.

100 uses of food (no super small carcasses or meat chunks): 2 or 2,000

Exploring
Chased lionesses
I sell chased lionesses in heat unless I have a reason to keep them in my pride. Once you hit the Dreamboat impression level, every lioness you claim in explore will be in heat.

Check the trading center for prices. List for higher if your claimed lioness has lots of markings, rare traits, or just generally looks interesting. Sometimes I give them names that match their appearance to make them more appealing. Sell them while they're in heat.

Sell for: 100-500

CRB NCLs
When claiming a lioness, you have a small chance of claiming a lioness who already has the cotton root bark (CRB) effect for her next litter, which increases the chance of a mutation. These are in high demand for players who mass breed for mutations, especially lethal mutations.

Include CRB NCL in the trade name.

Sell for: 2 to sell for sure, more if the lioness has wanted traits like a rare base or marking

Cave Materials
To build a cave, you need 10 large leaves, 10 large rocks, 200 SB, and an open cave slot (5 GB). I save up my large leaves and large rocks to sell as cave bundles. The rocks seem like they come up less than the leaves, but you have a chance of finding both in explore. (I believe in all biomes.)

Sell for: 1 cave bundle of 10 large leaves and 10 large rocks for 8+ (check TC prices)

Event Items
See what items will be in demand for the current event by checking the event wiki and event page once it opens. For example, in October I sold bundles of 50 amusement items for the Nirah storyline since they're needed to open the way to the heaven or hell shops. I sold several on the last few days of the month, so things like these are good for when there's a time crunch and people are last minute looking for 1 trade as a solution rather than tracking down all 50 items.

I may expand this to a monthly guide in the future, but for November I would consider making bundles of craftable items based on what they would be used to craft. I'll probably do this but I'm focused on getting the craftables I want for the citron base applicator right now.

You can bury items that will be in demand later in the year in your horde.

Games
Serengeti Shuffle
Score needed: 3500+
If you get above a certain score in Serengeti Shuffle, you can win more valuable prizes. These are the ones I sell. If you have the time you can replay until you hit that score. I keep some of these items, like snake scent, for myself.

Dove Feast: sell for 2

Black Stallion: sell for 2 each on TC but price may go up as we get further from October when they were in the event shop

Giant Tortoise: sell for 2 each on TC (they go for less on branches for some reason)

Whack a Snake
Score needed: 500-999

Giant Tortoise: 2