comsec rang me

Placed a reatively large order for a stock I already owned at the last price..Then attempted to sell my existing holding to myself. (at market sell order)..To take a loss(several grand) and reastablish at what I considered was the bottom..huge volume at this moment.The computer wouldnt accept the sell order...next thing the phone rings...They said I cant do it that way..Funny as this stock had a volume of 4 million yet not that many sellers and the price only went up 2 cents..somebody had managed to buy 4000000 shares that simply were not on the spread....This was one of my experiences this week and just thought to share it you all...