Can you please explain to me in detail how buying and selling for profit works?

I quit my job recently, I could not continue to work there with the ethical dilemas and constant fighting going on. It was good (profitable) while it lasted, but screw that place.

I now have a fair bit of money, but would like to make some more so that I can raise my savings to a nice round number, and have been wondering about buying and selling.

I know the basics, you buy in bulk cheap and sell individually at a profit, I know the dangers of money getting tied up in stock, and generally have the basics down, but how does it really work?

What are some good things to buy and sell?

Is it worth getting a shop or better door to door or online?

How do you maximise profit?

Anything else I should know?