Why doesn't an order type exist like: > "I'll pay the number shown, or something close to it"
? Like: acquire/lock all units to fulfill me market trade and if it is less/more than what i expect, cancel.
A couple markets have had flash crashes down to pennies. Imagine placing a market sell order right before that happens, selling your bitcoin for $0.15 instead of $15,000.
In reality you're usually somewhat safe, but market orders are always a small gamble for this reason.
Most people don't realize a limit order isn't "buy at this price" but rather "buy at the best price in my favor, but don't spend more than this much."
Fill-or-kill means don't put it on the books, just get it now or cancel. You can also typically specify whether it is a partial fill-or-kill too (get it all or none at all).
Of course that doesn't stop low-latency firms exploiting information differentials to get their bids in before your order makes it to the exchange, a process called front-running. It would be illegal for your own brokerage to frontrun you, but pretty much all discount brokerages sell information to third parties, which enables those third parties to front-run. Ever wonder why your Robinhood trades are free?