Someone posted that shipping charges are high and a detriment to selling guitars. Actually, it's all about knowing the "inside secrets". I just shipped a large electric guitar (box: 42x18x6) from central Florida to New York for just $23. (PirateShip.com) and I pay about $6 per box - but only on about 1/4 of my sales. Most guitars I ship in reused boxes from new or used guitars I've bought online.
Yes, I said "new". I sometimes buy inexpensive new guitars, mod them, set them up, etc. and resell. I have enough local sales to get "free" boxes from the new guitars, which I use for shipping.
In my spare time (which isn't a lot) I'm posting all my tips and stuff I've learned buying, selling and repairing guitars, on the GuitarFlipper website.