Trade with knowledge and discipline.Knowledge is the ability to derive risk and reward.Discipline is to take rational decision.
When your risk is more than reward, it is gambling
When your reward is more than risk it is business.
Stocks can be sold when you do not have them with the intention to buy later. Within a day in the case of intraday.
Bid is he buy order and ask is the sell order. Bid will always be in descending order and sell will always be in the ascending order