Large item sold on ebay what should i do?

Was eager to get rid of my guitar as I dont play it and dont itend to. Wanted to get money for it but got rid of the box and I went ahead and listed the item thinking how hard can it be to find a box. :S i cant find one. Two weeks after winning the item the person still hasnt got their guitar because i cant find a box. I have a feeling the transaction will have to be cancelled but i dont have the 75 quid in my bank so the person will have to wait till i get paid. Can i get the post office to box it for me???