A 320 FLC RPTED THE ACFT SLID BACKWARDS AND IMPACTED A BAGGAGE CART WHILE PARKED AT THE GATE.

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ACFT THAT LANDED BEFORE US RPTED BRAKING ACTION POOR; SO WE SELECTED AUTOBRAKES MEDIUM. TAXI-IN BRAKE TEMPS WERE RISING PAST 500 DEGS C; SO I HAD FO CALL AHEAD TO LET MAINT KNOW WE WOULD NEED TO CHOCK ACFT AND RELEASE THE PARKING BRAKE. MAINT SAID THEY WOULD PASS THE WORD. AFTER PARKING; I TRIED TO USE HAND SIGNALS TO MAINT MAN TO BE SURE WE WERE CHOCKED; BUT HE DID NOT UNDERSTAND. SO; I OPENED THE WINDOW AND VERBALLY ASKED IF WE WERE CHOCKED; HE SAID YES AND WALKED AWAY. I RELEASED THE PARKING BRAKE AND CHKED THAT WE WERE NOT MOVING. AT ABOUT THAT SAME TIME; THE EXTERNAL PWR LIGHT FINALLY CAME ON; BUT WE HAD ALREADY STARTED THE APU; SO I TOLD THE FO TO LEAVE THE APU PWRING THE ACFT. WHILE OUR ATTN WAS ON THE PWR; THE ACFT STARTED SLIDING BACKWARD. WE BOTH SAW THE JETWAY MOVE FORWARD; AND THEN REALIZED WE WERE MOVING. I STOPPED THE ACFT WITH FOOT BRAKES AND RESET THE PARKING BRAKE. WE HAD MOVED ABOUT 3 FT. THE BAGGAGE LOADER WAS AGAINST THE ACFT AND PUNCTURED A HOLE IN THE SIDE. I LOOKED AT THE NOSEWHEEL AND SAW WHERE THE SKID MARKS INDICATED THAT THE CHOCKS WERE SLID THOSE 3 FT.