First you must know if the seller will accept offers, to do this, while in their listing, scroll down to the tab that says: "Product Info./Shop Policies", here you will see if they accept offers on the product of now. If they do, to make an offer on a sellers closet item, click on the Send Message button in their listing (It will popup a box to write your message and your offer price) be sure to mention that you would like to make an offer on the product. Please be advised that they may or may not accept the offer.
If they accept your offer, they will go in and edit the price of that item, be quick as someone else might purchase it before you.