Where can I sell things online for free?

Be aware that there are always going to be some fees involved when selling online. Some platforms charge listing fees upon adding your product, while others use transaction fees or otherwise take a percentage of your profits when the product sells. Even if you don~t need to pay anything else, payment processors are also always going to take a small cut of each transaction.
The best strategy is not to try to completely avoid fees, but to cut down on them as much as possible. Use an eCommerce software provider with no added transaction fees, for your own website. If you sell on eBay, be ready to take advantage of promotional free listings that may be periodically offered to you. Other marketplaces may have their own ways of reducing listing fees for certain sellers.