Cryptocurrency trading usually happens on an exchange and the exchange almost certainly won't make any on chain transactions every single time a trade occurs, t

Cryptocurrency trading usually happens on an exchange and the exchange almost certainly won't make any on chain transactions every single time a trade occurs, they will just swap the balances around inside their own internal systems. The on chain transactions will only happen when a user withdraws their currancy from the exchange. So trades certainly can be very easily rolled back by exchanges up until the point that one party withdraws from the exchange. And during major instability and crashes it's not unusual to hear that the exchanges disabled withdrawals temporarily. I assume this is at leaat partly until they have chance to asses if any action is necessary.