For buyers - you sign up, buy credits, browse through all the cards they have, searches etc... make offers or just buy them straight up... pay with your credit... u batch up all your purchases even though they are from different sellers, send them all in one go to your home saving postage...
For sellers - you sign up, send your cards in, pay a fee for each card being processed, they scan every card, post them on the site for you, if u get a purchase, u get store credits which u can turn back into cash but they take a cut... if u re-invest store credit, they take no cut... if u keep your cards on the site, they charge u a monthly maintenance fee