> they get to sell it right away

That's not true, it just seems like it's true. A stock market matches people who are offering to sell a stock with people who want to buy a stock. But it's entirely possible that you elect to sell a stock, and no one wants to buy it, so it won't sell.

The price that you see on a stock market is actually the price of the most recent transactions of that stock. When you actually go to sell a stock your software will ask you what price you want to sell the stock at. Often it will offer a pre-filled in price range that you might want to sell the stock at. Most people just click OK, but you can change these numbers if you want to. You can it to to ask for more, or ask for less.

In general when the price of a stock is falling there's a BUNCH of sell orders for the current price that are going unsold. It's the people who are willing to sell for less than the current price that are getting their trades completed and that's why the price keeps falling over time. Because there's more and more sellers under the current market price.

If you look at a website like this one https://www.cboe.com/us/equities/market_statistics/book/aapl/

You'll see a real time list of "bids" and "asks" for Apple Stock. The "bid" is the maximum price that a buyer is willing to pay for a share of stock. An "ask" is the minimum price that a seller is willing to take. When a bit and an ask meet at the same price, a transaction occurs and the stock is bought/sold.

Lots of people put in sells or buys at prices other than the current market price in hopes that someone desperate comes along and they can complete the transaction. But most of the time these orders just expire without a transaction ever occurring.