Trading is executed by exchange through brokers. One should enter trading with trade plan. There are different types of trading like day or positional trading. Validity of these orders are accordingly decided. Its possible to sell a stock before purchasing it, but its only applicable to intraday trading. One can buy shares at BSE and sell at NSE and vice versa. Stock trading is not a gambling but a business where one enters trading with knowledge of risk involved and possible returns generated from this risk.