I'm assuming you are in US. It depends on the source of the income. Since you mention "sales" and you are on the startup stackexchange, it probably falls into a reportable income category.
If you are selling junk from your house on Ebay or Craigslist at a loss, it is not reportable; aka Garage Sale, Yard Sale, etc. This is only if it is a non-business activity (i.e.: not a systematic activity with intent of making profit).
See IRS tips: Online Auction Sales
If you are selling phone apps or getting ad revenue, then you need to report the income. You also need to determine if you are a hobby or a real business. If it's a small amount, It's easier to report it as hobby income, but you won't get to deduct (most) expenses. If you are trying to build a real business, report it as business income, but then you need to fall into the requirements for a business.
See IRS tips: Hobby or Business?