Simply search the items for sale and select your size. You can choose to Buy now at the lowest available listed price, or Make offer. When you make an offer, you can offer any price you like, and you will not be charged unless the offer is accepted.
If a seller accepts your offer within the time frame, your card will automatically be charged and your shoes will soon be on their way. At any point in time, if you get tired of waiting, you can always choose to Buy now and complete the purchase immediately.