When a person sells something, it means they give it to someone else in exchange for money.
Let's say you have a toy that you don't want anymore. You can decide to sell it to another person who wants it. The other person would give you some money in exchange for the toy, and you would give them the toy.
Selling can be done in different ways. You can sell things at events like yard sales or flea markets. You can also sell things online, by taking a picture of what you want to sell and posting it on websites like eBay or Amazon.
When people want to sell something, they set a price for it. This means they decide how much money they want to get for it. If someone else wants to buy the thing, they can decide if they want to pay the price the seller is asking for.
Selling things can be a way to make money if you have too many things or if you no longer want or need something.