If the handling fee is charged at 0.25 ‰, your calculation is correct. If both sides open positions and close positions, they will get money every time.
32 1 * 300-(2100 * 300 * 0.000025)-(2121* 300 * 0.000025), of which 21point is earned.
4. There are no restrictions on individuals. As long as you meet the conditions for opening an account, you can buy and sell at will within the scope of your own funds. Institutions mainly hedge their stock positions by shorting, and the proportion of positions opened is also limited, while individual investors do not have these restrictions and are much more flexible than institutions.
The stock index is T+0 trading, and it can be traded unlimited times every day. As long as you open the position, you can immediately close the position and sell it without waiting for 1 second.