in the past I have had issue with paypal over this, as a buyer they insist you have to cover the cost of return postage regardless of the problem, and you have to issue them with a tracking number, they then return you the original purchase price and original postage but not the return postage.

If you dont comply they wont refund the money.

All depends if the seller is decent and offers return postage cost or justs follows the paypal rules