I just added the following to all my ebay listings:
Whether you buy a machine from me or anyone you should verify that they are a member of the "Coin Op Preservation Society" (CoinOpPreservationSociety dot com). This is a group of dealers and collectors who prescribe to a strict code of conduct. Go there to see who are members. If you are thinking about bidding on a machine from a slot machine collector/dealer and they are not on the list you might want to ask them why.
I have also added some words on the home page of the COPS site telling people that if they are thinking about buying a machine from someone on eBay and they are now member of COPS then why are they not a member of COPS.