Not quite correct Clarry.  

Postage can be set to 'Freight - large and bulky items'.  Therefore delivery cost can be worked out once OP knows which State/postcode said guitar is going to.  

Should prospective buyers ask how much courier will be, OP could log on to a courier broker (eg Interparcel) and follow the simple instructions to provide a courier quote to X postcode..  @OP if you did go that way, do NOT select TOLL as they do not collect from private individuals, only companies.

I would also suggest OP make it quite clear in his listing that the item has to be sent by courier, as it is over AP's length limit.