I've put together a webapp where users can list their articles for sale with a price and the payment is done via paypal. Now caluclating the price for the payment won't really work if there is shipping cost involved and I don't have a good method for adding the shipping. I've implemented usecases for adding and listing an article and also buying the article and paying via Paypal which works, but I don't know how to do with the shipment since the seller does not know who the buyer is when listing the item. Do you have any suggestions how I can improve the solution?
See how Ebay solves this task:
It is byuer who selects shipping options. In most cases only country is required. This dialog could be accessible with