It may be the stickiest scam on the Internet — a nine-year saga of deceit that has seen thousands of altered postage stamps sold to unwitting collectors on eBay and other Internet auction sites. More striking than its longevity, though, is that the mastermind has never been charged with a crime, even though his identity apparently is known to eBay security, law enforcement officials and some of the nation’s leading stamp experts. The man believed to be behind the scheme is a longtime stamp dealer living in upstate New York. He has been investigated by law enforcement, suspended by eBay and exposed in Internet forums devoted to stamp collecting. Yet the massive operation continues to churn out philatelic fakes, burning collectors and, some say, undermining the very foundations of the hobby.