Do one need to pay tax for online side hustles? E.g. Selling online courses/subscription fees collected/sales of e-book?

If you earning above S$6000 a year, by the law, you will be required to file your taxes. More information can be found here: https://www.iras.gov.sg/media/docs/default-sour...

It's super grey.

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For example when you sell a used item on carousell for higher than purchase price, is that 'profit'?

By right, yes.

U go read iras website, no need to think so much, any income all must pay income tax, it's black and white, all the comments on whether cash, can trace is just whether you want to siam or anot. hahaha.