I think it is a business when you are showing a profit. So, if you are reselling your own possessions which depreciate in value, you are not making money, but if you are scouring yard sales and reselling those items at a markup that covers your expenses and gives a profit, then it could be considered taxable income. Likely your jurisdiction will have thresholds for which you must register as a business for sales tax purposes, instead of just claiming the profits as personal income.