I disagree with Stehno, in as that I do see a benefit to having a middle man. I know I was really sweating out sending $8,000 to someone for their speakers. Luckily for my sanity, he had not taken them home from the stereo shop yet, so I was able to contact the stereo shop and they informed me that they were indeed holding them there. When I bought my used amp for $2,000, I had to completely rely on the seller to be honest and send it to me.
We sometimes get used to sending money blindly to people. I sometimes think we are too trusting, buying things without seeing them and sending money to complete strangers across the country, or even out of the country. I think we start off small, buying and selling records, cd's, then moving up to cables, then to amps, and then to speakers (I speak from my experience here), and we never stop to think "What if this person is really dishonest, and has actually sold this same fictional cable/amp/speaker a dozen times in the last week, and will be disappearing like Houdini!"
I personally would be willing to pay a small fee to have an intermediary who would hold the payment until the item was delivered and then shipped out. It would only take one really bad experience for me to be completely turned off by the method we currently use to sell our used merchandise.
I would not be wanting the middle man to be any more than a escrow holder/warehouse actually. (They don't need to be an inspector). Just someone who can connect a face with an email address and to give me a bit of reassurance that the person who is going to send me their used multi-thousand dollar piece of equipment actually will.
My thoughts on the matter, anyway!