system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 : How do stocks work ?

If you had a lot of money, could you not buy a bunch of shares of a (usually stable) stock, which would cause the share price to rise (cause of a surge in demand caused by all the shares you just bought) and then sell your shares at the new price, making a bunch of profit. Rinse and repeat.
gpt
As soon as your big sell order hits the market, people see that and the prices offered to buy will start to drop.

If you split your order in multiple small orders then the execution of those will similarly make the price start to go down. 

You can't just buy or sell an arbitrary amount at the current price, only whichever volume is on offer at that price at the moment.

So what doesn't happen is:

Step 1) Buy 1 million shares at 10 USD.
Step 2)  Price goes up.
Step 3) Sell 1 million shares at 12 USD.

It's more like:

Step 1) Try to buy 1 million shares at 10 USD.  You get some at 10 USD some at 11USD some at 12 USD some at 13 USD some at 14 USD and some at 15 USD.
Step 2) Try to sell at 15 USD?  You get some at 15, some at 14, some at 13 etc.

Net profit: Nothing.