Those guys mostly don't know what they are looking at either, Jeff. They rely on image quality and printing and attribution notations. With big money coming in, definitely an issue how photos are dated. I mean, a thief might not bother to forge a press release slug, or press and printing notes on the back of a $20 photo, but on a $2,000 photo, yeah, that tracks. The other issue is that some of the 'best' photos are not Type I as PSA defines it. PSA says Type I are the best. Team issues are Type III:
I'd rather have a 1960 team issue Chamberlain than a random original photo from the same era.
What is the scourge right now are the scanned and printed fakes on eBay. I've bought a few of those on eBay. Run a search for a vintage photo subject in sports and you will come up with tons of cheap knock-offs. The good news is that they are obvious in hand and from the price. The bad news is that when they are sold as originals you have to get them in hand. The worse news is that those doofuses who buy them thinking they are genuine and don't know better then drip them into the resale market so that even if the maker discloses he printed it the next seller puts it out as an original.