I have seen this discussed here, and the eBay experts will, I am sure, tell me if I am wrong, but I believe that contacting bidders on an item would be considered interference with an auction, and therefore against the rules.
There is, however, the option of informing eBay about something that you believe is being fraudulently represented, and the eBayologists on here will know exactly what it is, or you can hunt for it in the voluminous labyrinth of eBay help, I would probably do that live help chat thing, if you have a computer modern enough to use it!