I think it can work in favor of a buyer. With some people only buying stuff with a team letter/well respected dealer ect. It leaves open nice bargains on stuff a collector/small dealer is selling on his own. I mainly collect common type player's game used stuff. I still check images, tagging and make sure everything is correct for the jersey... ect. Can pick up some nice stuff that way cheap. I don't think people are buying authentic jerseys, making the correct tags/or removing them and putting them on from other gamers/game issued to lose money on selling a common player's game used jersey. I feel more comfortable buying these types. Not having a COA on these doesn't bother me, as long as everything is correct with the item. But if I were to send big $$$ on a HOF type deal. I would look to buy one with as much paperwork/photos with it. That is why you see the NFL/MLB type auctions on gamers go for so much.
I think some COA that are done right are the best thing for the hobby. I own a few NHL game used jerseys and a helmet from Meigray. With there serial # tagging, LOA and the population report on there website showing the jersey serial # ect is about as well done as it gets. I have no question these are the real deal. I even have a Getty photo match from them on one jersey showing a match up on a rip in the jersey. Showing the date the photo was taken and where. They also keep track of when each set was used. Makes it real easy to look up pictures and info of what your player did in your jersey. I also own a couple things from Ballpark Heroes. I like that they put a few pictures of the item in the letter and sign it.