Just like negotiating with someone at a real marketplace, you can make an offer to a seller if you want to try to get the item at a lower price than it's listed for.
To get started, just click the "Make An Offer" button on the listing you're interested. If this button is not available. this means that the seller of that item has opted out of accepting offers on their listing:
Once your offer is submitted, the seller will have 24 hours to reply to your offer. They can either accept, deny, or counter your offer, at which point you'll receive an email notification with some next steps.
If the seller counters your offer, you'll be able to go back and forth in your negotiations until somebody's offer is accepted, once you've mutually agreed on a price.
Once an offer has been accepted by either party, the buyer has 72 hours to submit payment for the item.