There has to be another party on the other side of every trade. If someone wants to sell Meta stock for $300, there needs to be someone who wants to buy it for $300.

Exchanges have a collection of their participant's bids ("I will buy for this price") and offers ("I will sell for this price"). When a bid is matched with an offer, the transaction goes through.

Now this is the important part - there are only so many people willing to buy for $300. If there is a mass sell off, all those offers are consumed, and the exchange starts filling the offers of people who would buy for $299.99 (or whatever the next lowest price is). Once those offers are consumed, further down the price ladder we go.

That alone will ensure that not all sellers are getting top dollar, however a market selloff like that would trigger a cascade of lower prices (buyers withdrawing their bids and relisting at a lower price, or not relisting at all), along with things like stop losses and algorithms getting triggered.