"Authentic vintage movie posters are valuable pieces of memorabilia sometimes worth thousands of dollars. But Haggard wasn't a legitimate purveyor of fine collectibles. He created and peddled fakes."
As for how the fakes were made, it seems as though Haggard, who lived in Georgia, had been using a printing company in New York to make high-quality duplicates of digital scans of the originals. The article doesn't say how the FBI came to be involved, but I suspect that once you start dealing with posters that normally sell for thousands of dollars, eventually you're going to sell to someone who actually knows what they're supposed to be looking at.