Wait. Whats happening? People are trimming the cards to get a better grade? Wouldn't PSA or BGS be able to tell? How would you do your homework - via that site you posted? Does it say "watch for trimmed cards"?
Probstein and PWCC don't trim the cards themselves but they do allow people to funnel trimmed cards through their auctions. Vintage gets it the worst. Countless examples have been documented on the blowoutforums. I wouldn't touch graded vintage right now.
The big names are the most known to get trimmed but some of these guys trim EVERYTHING, even relative no names. That doesn't mean everything is trimmed or everyone who submits to grading is trimming their cards. You just have to due a little homework on cards you're looking at, especially numbered cards since there is a good chance there's a trail to the original listings. The two sites I linked allow you to search for old listings and they go way back.
There's certain brands that should raise red flags when you see PSA 10 listings. Most National Treasures, Immaculate and Exquisite cards in Gem Mint holders make me raise my eyebrows. They are notorious for chipping around the edges and soft corners.