I put out a request on the Gear Wanted page for a Korg Polysix. I got several helpful leads, so thanks to all who replied with ideas. In particular I would like to thank Z for his reply to my post and for his part in helping me to procure a Polysix that he had just had serviced. He posted many excellent photographs of the synth, including a scan of the receipt from the tech who did the work on the synth which detailed the work done and the price of repair. Z also made a great video in which he tested all of the functions of the Polysix for sale. His communication was top notch. He replied quickly and professionally. When I asked him to please pack the Polysix well for shipping because I was concerned about how UPS handles packages (I once bought a Juno 6 on Ebay and when I received it, about 20 of its teeth were punched out in the middle of the keybed) he directed my attention to a another video he had made in which he shows how to properly pack a synthesizer for shipping
So, long story short, Z is an awesome dude and you should feel very confident when purchasing gear from him.