Just spent way too long mind boggled on this question. Thanks for all the detailed explanations. I can confirm that the below is the exact working from GMATPrep....
"The sum of the integers in list S is the same as the sum of the integers in list T. Does S contain more integers than T?
(1) The average (arithmetic mean) of the integers in S is less than the average of the integers in T.
(2) The median of the integers in S is greater than the median of the integers in T."
....Clearly, it does not state the positive / non-negative restriction. Is it safe to say that GMATPrep dropped the ball on this one?