Is puzzling (no pun intended).
I found an interesting 2016 Upper Deck Captain America 75th Anniversary Vibranium Shield Mosaics VS-20 card for sale on COMC, with a marker line on the front of the card.
That card is listed for $0.65 with a "marker line on front" condition note. That line was created by one of the following: the buyer marked that card and sent it to COMC; or Upper Deck's printer malfunctioned.
I've never seen a trading card with an ink mark like that before. What do y'all think caused that mark?
Update: Someone purchased that card and relisted it for $30.25.
Images courtesy of COMC.com.