I'd like to THINK that if you could show eMails - a hole in your bank account - service receipt from Canon - EXIF data that shows the lens has been used on your camera etc then in my opinion it becomes one of those "if it looks like a duck - walks like a duck - quacks like a duck then ... it's probably a duck" type situations.
And if the member won't cough up a receipt, just let me know who they are and I'll put a colour cast on all their photos - blur them - and rotate them 0.5 of a degree! (just kidding!)