Do you need to pay income tax for profits from carousell, flea markets, online marketplaces, etc?

Revenue is not equal Profit.

Profit is revenue minus costs (R - C = P)

If you are selling secondhand items where the cost of the goods is actually higher, then in accounting, you are not turning a profit, you are running a loss. (hence no income tax needed)

Unless you are running a 2nd hand business where you actively buy goods from others (like 2ndhand computers, secondhand furnitures, secondhand bags etc) and then sell it to others where you are making a profit, then you should document it down.

First $20,000 individual income in each financial year is 0% tax rate, so you don't have to pay any tax if your business is generating less than $20,000 profits.

Nope, I think selling pre-loved items shouldn't be taxable.