Let's say you are a private company and want to raise money. Instead of taking on debt you decide to sell some of your business. You will go through an invest

Let's say you are a private company and want to raise money. Instead of taking on debt you decide to sell some of your business. You will go through an investment bank who will help you quantify the value of your business and underwrite/distribute shares of it to the public. This is the primary market where new shares are created. After that happens market participants can sell those shares to each other based on supply and demand. As demand for a companies shares rise, prices rise and vice versa. The secondary market is what you and I as individuals think of when we talk about buying and selling stocks. There is no real discount that Apple could offer Berkshire because they aren't selling the shares, market participants are (individual investors, brokers etc). The trade would take place as a complex series of transactions on a variety of exchanges. Sometimes Nasdaq, but other times on exchanges like ArcaEdge, Bats, Direct Edge, NYSE and others. The complexity is that if you dump 1 Billion dollars into the market to buy x number of shares of AAPL you will drastically increase demand and move the price considerably, so the trade must be executed as a series of transactions over a given time period. Now if one public company buys another public company in a merger there is a set share price that is negotiated but that's outside of the normal process of buying and selling stocks.