Wowza got some complicated answers but they are all right. Heres an example of makeup swapping.
Its like lets say you have an eyeshadow that you regret buying, so you post the eyeshadow up on a website and say you write that the eyeshadow is up for swap. And lets say this lady messages you saying i want your eyeshadow in exchange for my lipstick (And its an lipstick you wouldnt mind having). Then you agree to swap an eyeshadow for a lipstick, most people like to swap for an item thats simliar in value. And you can do multiple items as well.