What do I put in "What product or service did you provide?" under business expense if I am full time day trading stocks?

As a day trader, since your time horizon is only within the day, you are a reseller of securities. It is assumed your goal would be to buy low and sell high within the day, or the reverse; sell high and buy low within the day. For more information on this subject: TurboTax -

Capital Gains When Day Trading in Canada

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