I was wondering what exchange will let me buy/sell bitcoin quickly so I can do day trading.
Many exchanges are available, but not all exchange platforms are recommended. I personally have been using bittrex for a long time and I've never had a problem there. You just need to enhanced your bittrex account level by giving more identity like selfie photo, ID card etc. That way your bittrex account will be safe and will not be disable like most cases of users who have problems with their bittrex account.