Many people occasionally sell items online through auction websites such as eBay or online classified advertising services such as Craigslist. These sales are the online equivalent of garage or garage sales. Generally, when you sell old personal goods (such as old clothes) online, you incur losses, meaning you receive less than what you paid for them. Losses from the sale of personal goods (such as clothing or cars for personal use) are non-deductible personal losses.
The IRS doesn't expect you to report them on your tax return. A casual seller selling an item at a profit would have a taxable capital gain that is supposed to be reported to the IRS (see the collectors' or investor analysis below).