Who pays for shipping is entirely up to both of you. We always advise clarifying shipping methods in the offers dialogue or in the public questions before agreeing on a price. Most of the shipping companies offer a price calculator on their websites (for example Royal Mail), where you can inform yourself beforehand about the incurring shipping charges. The costs, of course, depend on the destination country, as well as weight and size of the letter or package.
You also have the possibility to choose between uninsured and insured shipping. We always recommend insured shipping, especially for high-priced items. Insured shipping is a little bit more expensive but you will be provided with tracking ID, which means, that you will be able to track the item while it is in transit.
When using the Buy Now feature (App only), the shipping costs have already been specified by the seller and will be added to the total amount paid by the buyer.
Also please make sure to always keep the shipping document in a safe place until the item has reached the buyer.