Beagle,
Could someone buy a "dodgy" part, believing it to be genuine, if they stuck to reputable dealers, or are you saying that the dealers themselves may be duped?
Following the AA plane crash in Colombia, it was estimated that as many as 523 parts could have been removed, and be available on the second-hand market - "counterfeiters descended on the crash site, stealing aircraft parts before all of the 159 bodies were removed. They used sophisticated tools to extract major aircraft components, including cockpit avionics and engine thrust reversers, and flew such parts off the steep mountainside using helicopters"!
An article appearing in 1997, makes grim reading:. . <a href="http://www.bmpllp.com/Articles/IP-Aviation.htm" target="_blank">http://www.bmpllp.com/Articles/IP-Aviation.htm</A>