As mentioned, the biggest issue is with rogue sellers/hijacked accounts where the backs are listed at very low prices and obviously troll for suckers.
As regards bad buyers, I would definitely put a provision in the eBay auction that no zero feedback or unverifiable bids will be accepted. Similarly I would be explicit about STRICT payment & shipping options and don't budge on following this through. Don't be surprised if you get a few follow up emails from fraudsters if you don't sell the first time around - genuinely interested buyers will use eBay's follow up email service and not outside of eBay.
In the listing photos I would also follow the previous guidelines about making the presentation professional, clear and also add some background material that includes your eBay account ID and even date. This will prevent fraudulent sellers reusing your material later plus will also signal to potential purchasers that the auction is real and not a fraud.
Prices for 645M are much lower than the $4-5k mentioned here. You may be lucky but the going rate is closer to half this amount. Check with secondhand brokers like KEH and you'll be shocked at the value of these today. Hasselblad and Contax mount backs hold their value slightly better for no other reason than they are arguably harder to find.
I recently sold 2x 645M's privately but at nothing like $4k each. (average was $2.5k) One thing that is vital is to include the Mamiya/Kodak view screen as these are no longer available. I do know of a dealer in the UK (TeamWork Photo) who can provide a custom view screen if required. I bought both of my 645M's on eBay from very obviously reputable sellers and we verified each other over the phone/email so that there was no mis-understanding or mis-trust.
To make it sell you need to put together a complete set of back shots, including the original Kodak box & cable sets, plus a shot of base with the serial number (legible!), image of the rear with the actuation count and a realistic explanation of the backs usage to date. If you price the back with a reasonable reserve then I think you'll be surprised how many genuine purchasers are out there.
Finally: It's a great system at Canon 5D level of money but you're not going to get 1DSII type of money any more. Sad but true.
