How does a mark affect a card's grade?

[Kutcher55]

Here it is. All that stuff around Rick's collar isn't actually a mark, even though it looks like it could be made by a pen. I believe the MK is due to the staining on the back. Which is kind of ridiculous because if that was worthy of a mark then half the bowman cards made between 1951 and 1955 should have "MK." When it comes to the MK qualifier in particular, not all marks are created equal and the graders will often be confused as to what to do, but ultimately the market will decide how much the mark matters to the pricing. Obviously a high grade with a MK will tend to take the biggest hit in pricing because people aren't as likely to spend top dollar for a card with MK. But there are probably exceptions even to that....