I have bought and sold a fair few guitar on Ebay, but I always do it on my terms- Cash/collection if at all possible, never post anything uninsured- especially abroad!
The best thing is when you put something up with clear indications of any changes and you get people just being rude- I had an SG with Hiscox and graphite saddles and had a chap offer me a ridiculous lowball figure because I had 'devalued' the instrument. I told him to shove it and got the full asking price a couple of days later..
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PSN: stickyfiddle