When you resell something, you sell it after you have already bought it. You are not making it, and you are not the first person to offer it. A shop buys stock and resells it to you. A person buys a concert ticket, cannot go, and resells the ticket. People also resell phones, bags, games, and cars they have used. It is an everyday verb, not a fancy one. Though actually, in talk about tickets or limited shoes, resell can sound a bit sharp, because some people buy only so they can sell again at a higher price. The past is resold, not reselled. The matching noun is often resale: if something is for resale, you bought it in order to sell it on.