As a U.S, Citizen you are require to report income for all sources on a world wide basis. This includes items you sell on eBay. That being said, items you sell around the house are probably sold for less than you paid for them and that would not result in any income. However, when you set up a eBay store and sell other items the difference between what you sell an item for and its cost less your other operating expenses will possibly result in a profit and cause tax liability for you. This is considered self employment income.
Any individual seeking legal advice for their own situation should retain their own legal counsel as this response provides information that is general in nature and not specific to any person's unique situation. Circular 230 Disclaimer - Advice given in this response cannot be used to eliminate penalties with the IRS or any other governmental agency.
As a U.S. Person, it is your duty to properly report all worldwide income and to pay all taxes due on such income. This applies to both situations you mention. If you open a store, you may also have to pay business license taxes in your city as well. Also, be sure to verify sales tax is properly collected and remitted to applicable states.
You are self-employed if either of the following applies to you:
-You carry on a trade or business as a sole proprietor; or
-You are a member of a partnership or limited liability company that files a Form 1065, U.S. - Return of Partnership, that carries on a trade or business.
Note: You are also self-employed if you have a part-time business, in addition to your regular job.
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