The people who quote both sides
A market maker posts a price to buy and a price to sell at the same time, and stands ready to do either. They have no view on Bharat Foods. What they are selling is the ability to trade *right now* instead of waiting for a natural counterparty to appear.
The gap between those two prices is the spread, and it is how they are paid. Buy at the ask and sell immediately at the bid and you lose that gap — it is the fee for immediacy, and it is charged whether or not anyone calls it a fee.
What immediacy costs
Drag the spread. It is money, and it comes out of your position the moment you enter.
A liquid share has a spread of a few paise. An illiquid one can cost several rupees to enter and leave — before any brokerage.
The spread is not a fee anyone charges you. It is a fee you pay anyway.
The others in the queue
Arbitrageurs exploit the same asset being priced differently in two places at once — the cash market and the futures market, or two exchanges. They are not predicting anything; they are collecting a difference that should not exist, and by collecting it they remove it.
Proprietary desks trade a firm's own capital rather than clients' money, which frees them from mandates but also from anyone else's tolerance for loss.
High-frequency traders do a version of all of this at speeds measured in microseconds, competing to be the one who fills your order. The honest summary is that they have made spreads tighter and made the market harder to read in equal measure.
You are not competing with these people, and you cannot beat them at what they do. You are also not trying to. They compete over microseconds and fractions of a paisa; the questions in this course are about years. Speed is only an advantage in a race you have entered.
You buy 100 shares and sell them a second later, with no news and no price move. What happened to your money?
So far, everyone here decided to trade. There is a growing group that buys and sells without deciding anything at all.