To sell an item, drag it out to any available space. Some items go on shelves, some items go on walls, and some items go on both, while bigger items tend to go on the floor.
Once an item is out for sale, it will take some time to find a buyer. Some items will sell quickly, while other items, especially rare or expensive items, take longer.
Once an offer comes in, you've got a choice to make. You can take that offer and pocket the cash, or wait for another offer to come in. New offers come in every 30 minutes. Be careful, though: new offers aren't necessarily higher, so it's up to you to keep track of the value and how much profit you're making.
In a hurry? Entice in a buyer with candy and get an offer now, or get a new offer immediately if you already have one.