Though you have to feel for the publisher, but one the other hand there has to be a reason why they are buying used and as a publisher it is there job to find out why they are buying used instead of new and fix it (Very simple). They have already found ways to get people to buy new like add more mp maps that make it a better value.

  What they could do is lock up the mp on used games so that the new is a better value, or just go with digital distribution and that will end used games.