Just a guess but someone marking the cards would probably destroy much value.
They probably marked the sleeve holding the card.
Public Service Announcement for lurking DUmmies.
I don’t know how this relates to trading cards, but when it comes to comics the grading service is very, very particular. They have to be to retain their reputation. In other words, with an item that a normal person would consider in “mint†condition the grading service would probably find some sort of flaws/blemishes that would put it below that. Once graded, the item is slabbed (sealed in a hard, clear container).
With comics, where the grades used to be words (good, fine, mint, and so on), the grading service uses a number grade ranging from 1 to 10 with 10 being an item in pristine condition. It is extremely rare to have something that grades out as a 10.
Once something has been graded and slabbed by a legitimate and reputable grading service it will usually sell for more than a similar uncertified item.
Also, the prices you see in price guides are usually for items in mint condition. In addition, just because something is listed in a price guide at a specific amount it does NOT mean that you will be able to get that amount from for it.
Again, the above is based on comics (from knowledge that is 15 or so years old). Things may have changed and may not even relate to trading cards; however, there is one thing that is true now that was true 15+ years ago. Comics and trading cards should generally be bought to be enjoyed. If you do so you will usually get your moneys worth, but if you buy with the intention of getting rich you will often end up disappointed.
Finally, any grading service is in business to make money so they will charge for their service. Once you figure that into the equation you could end up with a loss when and if the item is sold.