Good question. COMC does its best to make sure every item sold is real and unaltered, but some slip through the cracks. I have reported hundreds of fake or trimmed items to them and they've removed them from sale (or reversed the sales) in those cases. They have a very liberal return policy for an counterfeit or altered cards that they've sold, and that includes ones in graded holders that got past PSA, BGS, or SGC.
They now charge the original submitter $10 for every counterfeit or altered item that was submitted to them and remove it from sale from their site. If you ever have a question on a card that is listed for sale, you can mouse over the picture, select "Report Error" in the bottom right corner, then click the "Questionable Authenticity" circle and explain why you think it's a fake. Or you can email them at
staff@comc.com if you're thinking about buying it.
COMC only charges a few quarters to scan and list each card, so they're not thoroughly checking every card coming through the submissions. And their basic intake staff doesn't have the knowledge or accuracy to get everything right. But they're trying.
Right now there are a bunch of 1991 Topps Desert Shields on site. There are a lot of those being faked right now that are making it through PSA and BGS graders, and I'm sure some of the ones on COMC are counterfeit stamped as well. I've personally found a bunch of the 1947-66 Exhibits that were counterfeit, marked them as such, and they were removed from sale and/or the buyers were reimbursed.
They got hit by a couple of the professional scammers a couple of years ago who were passing altered cards in PSA/BGS/SGC holders through COMC to sell. Those sellers were mostly kicked off of COMC and COMC fulfilled all return requests, AFAIK. Again, going way past what other consignment services have done, and way past what many grading services have done. There are a couple of people on Net54 that work for COMC and may be able to give you more information. I'm just a guy that buys/sells on there and loves what they're trying to accomplish. I also make lots of recommendations each year on how they can do their jobs better, so they listen to me.