That was the Communists supporting Saddam, not Serbia. Had the UN not imposed crippling sanctions on Serbia and Montenegro, there would've been no need to trade weapons for oil.
Serbs supplying weapons to Iraq in 2002 have nothing to do with communists. And note that it was the extreme nationalistic Serb Radical Party that sent its leader to meet with Hussein in 2001, even after 9-11. The Serb Radical Party is still the largest party in Serbia.