especially if you have an ebay store. just sell something in the store at $0.01 as a "buy it now" item and make it a private auction. this will only cost you $.06 to list. if you're worried that someone might actually legitimately buy this then just jack up the shipping cost to an amount that you'd be comfortable selling the item for.
now, have your friend "buy" the item and check out. the final sale fee should be 5% of $0.01, or basically $0.00.
now each person can leave the other some positive feedback and having the private sale will prevent others from seeing that your friend bought a guitar for $.01.