Swapnice allows you to barter and pay, meaning that you can find an arrangement that works for you. Simply contact the seller via Swapnice, and then directly make them an offer, which can be both money and items for sale, to fulfill their listing. Then the seller will be able to message you and, once you've agreed, schedule to meet and collect your new items. Remember that your offer can be money, items on your sell lise, or some combination of the two.