https://www.investopedia.com/terms/m/marketvalue.asp
Market value definition:
> obtained by multiplying the number of its outstanding shares by the current share price
The reality of selling has nothing to do with it. It's just a math equation.
Also investopedia isn't exactly a high quality source.
In the stock market, if a company has ten million shares at $100 each we say the company is worth a billion dollars, even though that $100 price is only the most recent transaction. We say that because if price times quantity didn't match what the overall market thinks the whole company is worth, the price would quickly correct until it did.
"Market capitalisation or market cap is the market value of a company's issued share capital – in other words. the number of shares multiplied by the current price of those shares on the stock market."
Net worth is the same concept. Jeff Bezos net worth is calculated based on how much his assets are theoretically worth based on current fair market prices. He doesn't have to sell all his assets to determine his net worth.
People don't like the "lost $100B in market value" part, but it is accurate since market value is calculated on paper.
Market value is exactly what the parent asserted it is, and the source of truth for that is your bank. If you are long 500k shares of AAPL and the quote is $100 - your bank will not consider you to have $50mil liquid assets. They know(and you should to) that starting to unwind a position of that size will decrease the market value a considerable amount.
Maybe an example will help you out here.... Let's walk through that AAPL example. You are sitting on 500k shares.
Step 1: Order 100,000 shares sold at market.
Step 2: Observe your average sale price is more likely to be $90-95 than $100.
Step 3: Order another 100,000 shares sold at market.
Step 4: Observe your average sale price is more likely $90-91 than $90-95.
Steps after this: same result, diminishing prices resultant from your sale(s).
See a trend here? The market value of your assets is decreasing based on the market's perception of value, not on the mechanical formula investopedia fed you. Your unloading of shares is going to decrease the perceived value, which decreases the price you can sell them for.
tldr; investopedia's advice on markets is barely more accurate than urban dictionary's advice on word meaning.
So as crazy as it sounds $100B in market value can be erased by a drop in the market price of an asset. But don't confuse that with $100B actually being lost or investors realizing $100B in losses. But in any event the title of the article is accurate.
Apple traded 21,583,997 shares today. Aggressively selling 500,000 shares (less than $100 million dollars worth) might move the market in this stock by perhaps 1%, certainly not anywhere near 10%.