Surprisingly, I rarely see these cards with the back rubbed off. They really aren't difficult to find in nice condition, so having the trivia question rubbed off on the back will definitely hurt the value. Hard to say how much, but it depends on the condition of the rest of the card.
I'm sure that you would find someone who would like to build a low to mid grade set, who won't be concerned about this trivia question being rubbed off, but I wouldn't expect to get much for the cards.
By the way, at the top of the page, there is a Yellow Banner that says "Buy-Sell-Trade." Click on it to get to that section.