That said, learning the logistics of how to buy stocks and earmarking a small sliver of your investable assets to buying individual stocks online can be fun and profitable. And at worst? As long as you don’t put money you can’t afford to lose on the line, stock trading will be a temporary diversion that leads to limited losses, lessons learned and a few self-deprecating stories to entertain dinner guests.
In margin buying, the trader borrows money (at interest) to buy a stock and hopes for it to rise. Most industrialized countries have regulations that require that if the borrowing is based on collateral from other stocks the trader owns outright, it can be a maximum of a certain percentage of those other stocks' value. In the United States, the margin requirements have been 50% for many years (that is, if you want to make a $1000 investment, you need to put up $500, and there is often a maintenance margin below the $500).
Courtyard of the Amsterdam Stock Exchange (Beurs van Hendrick de Keyser) by Emanuel de Witte, 1653. The Amsterdam Stock Exchange is said to have been the first stock exchange to introduce continuous trade in the early 17th century. The process of buying and selling the VOC's shares, on the Amsterdam Stock Exchange, became the basis of the world's first official (formal) stock market.
Let’s take Apple (AAPL) for example, which is listed on the NASDAQ stock exchange. Apple currently has 4.6 billion shares outstanding, of which 4.35 billion are available to be traded (also known as the “float”). Using today’s closing price of $201.75 (July 11th, 2019), Apple has a market cap of $937.44 billion. That’s a big company! (By the way, market cap is a simple way to gauge the value of a company. If you bought every available share of stock, the market cap is how much it would cost you to buy the entire company.)
People trading stock will prefer to trade on the most popular exchange since this gives the largest number of potential counter parties (buyers for a seller, sellers for a buyer) and probably the best price. However, there have always been alternatives such as brokers trying to bring parties together to trade outside the exchange. Some third markets that were popular are Instinet, and later Island and Archipelago (the latter two have since been acquired by Nasdaq and NYSE, respectively). One advantage is that this avoids the commissions of the exchange. However, it also has problems such as adverse selection. Financial regulators are probing dark pools.