The laws vary hugely based on city/county/state/country and purpose of sale, but in my experience in the US you are almost always required to report income made

The laws vary hugely based on city/county/state/country and purpose of sale, but in my experience in the US you are almost always required to report income made from personal sales and pay income tax on it, and the person who bought it from you is supposed to pay sales tax. Sometimes there is a quantity exception, fewer than x sales and/or lower than y total value. With trades it is much murkier but in my experience not required. Not to say I do or that the tax man will ever come for me, but the laws are there.