if you had £0.00 in paypal and paid for an item costing £20, paypal would apply for £20 from your bank account to be paid into your paypal account then straight out to the other users paypal account. If you then sold an item and someone paid you £15 it would go from their paypal account into your paypal account. You then have the choice to withdraw from your paypal account into your bank account or keep the money in your paypal account. If you then paid someone £30 via paypal then yes im assuming they would take the £15 you have in your paypal account and apply for £15 from your bank account