If a customer does not accept a package containing a product purchased from Marketplace that was sent to them (this is called a returned or unaccepted package), the unopened package will be returned to Webshippy's warehouse, where the returned products will remain in your online store's inventory and will remain your property.
Naturally, if a customer order is received for one of these products, it will be fulfilled from your webshop's own stock.
The same happens if you have already paid for the products in the order, but your customer cancels the order before it is dispatched. In this case, the product will still be shelved, it will remain your property and will be shipped out of your stock on the next order for the product.