The bit of routine that actually executes trading is located at [FE8] 2D760, [FE7] 2B062:
0802D760 8813 ldrh r3,[r2]
0802D762 8808 ldrh r0,[r1]
0802D764 8010 strh r0,[r2]
0802D766 800B strh r3,[r1]
and if we remove those last two lines then nothing happens when you try to trade the items. This can probably be used to disable the trading of certain items.