Obviously, Dean's, Greg Morris, and Burbank all rate the cards they sell based on PSA's scale. If you buy a card on COMC, they will perform a condition review on the card for $2 and give you the expected PSA grade they believe it to be. Here is one I had done:
https://www.comc.com/Cards/Baseball/...graded/COMC/NM
I will be submitting the card to PSA soon, but I've done this with multiple cards and appreciated them looking at the card in the warehouse for me. In the case they find an alteration they didn't note originally, they have taken the card back for a return.