If you make a $10 sale, IRS may not be able to know, But if you make 100K sale, IRS will definetly know about it. And proft caused out sale is definetly taxable, so you can only pay less tax by keeping a record of all your expenses which includes:
1. Domain registrarion and renewal charges.
2. Hosting and software charges
3. development charges
4. Your computer and hard ware cost.
5. Even your internet connection charges can be tax saving.
6. You can simply hire your two minor children: For example, if you hire your two minor children and pay $6,400 to each of them, you save $3,968 on your income tax (6,400 x 2 x 0.31, assuming you are in 31% tax bracket). Your children would claim $4,400 as Standard Deduction and if invest $2,000 in Traditional IRA, would pay "0" tax on their income. This technique not only saves you income tax, but also $1,958 on your liability for social security taxes (12,800 x .153).
I may miss few other things that you could show as legitamate expenses out of the sale.
But yeah, capital gains are Taxable.