hi swagat,
u can always short in margin. but u cant covert to delivery unless u have that stock in DP as i told before.
eg,assume u bought 10 rel shares at 900(say) few days back, now if u short 10 shares in 'margin' at 1420 and it moves to 1500, now it is better to convert it into delivery so that u will be still in profit of ( 1420-900) per share.
remember that the sell price will be still 1420 not 1500
regd
Satish