Deductions & credits

Without the mark to market election, only your expenses will go on schedule C since the income is already reported on sch D. A negative sch C imposes no SE tax. Since 80% of daytraders lose money, without the mark to market election, your losses are still limited to $3,000.

From the IRS,  Day trading :

Traders report their business expenses on Schedule C (Form 1040), Profit or Loss From Business (Sole Proprietorship). Commissions and other costs of acquiring or disposing of securities aren't deductible but must be used to figure gain or loss upon disposition of the securities. See Topic No. 703, Basis of Assets. Gains and losses from selling securities from being a trader aren't subject to self-employment tax.

In order to file a sch C, you must be a business. From  Day trading :

To be engaged in business as a trader in securities, you must meet all of the following conditions:

• You must seek to profit from daily market movements in the prices of securities and not from dividends, interest, or capital appreciation;
• Your activity must be substantial; and
• You must carry on the activity with continuity and regularity.

The following facts and circumstances should be considered in determining if your activity is a securities trading business:

• Typical holding periods for securities bought and sold;
• The frequency and dollar amount of your trades during the year;
• The extent to which you pursue the activity to produce income for a livelihood; and
• The amount of time you devote to the activity.

From Small Business and Self-Employed Tax Center

Publication 535, Business Expenses

Publication 334, Tax Guide for Small Business, For Individuals Who Use Schedule C

@spencer00 I hope this clears it up for you. Sch C has no income, only expenses for those without the mark to market election.