You can if you qualify. I've traded in an out of positions within minutes. Just don't free ride. That will get you a restriction, but then again, nobody permits unfunded accounts to purchase or sell securities as we did back in 1990s and before.
Crap, I recall making a $100,000 trade back in 1991 unfunded, and went out of the country on business, totally forgot about it, got an extension. I flew home, had to get a cashiers check and fly United to San Francisco to hand deliver $100,000 to the clearing firm to avoid a 90 restriction and reported to the FRB, then fly home.
Those were they days. I was licensed with the NASD at the time, I could not afford to be reported to the FRB. Gotta make good on your commitments.