As above, feedback should only be left when you feel the transaction has been finalised, and I can assure you no seller will expect to receive feedback for a purchase unless the item has been received and all is well (most sellers in fact take receiving feedback from the buyer as an indication that the item was received and there were no problems that need addressing - and not to give you the wrong impression, as it would not be a common occurrence, but if a seller ever asks you to leave positive feedback before they will address any issues you might have, ignore the request 😉 ).