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.
Actually, some users with enhanced verification already activated have reported that their accounts were locked. I'm very wary of using Bittrex right now. I think all these complaints will blow over, but right now, it's not worth the risk.
With the legally precarious situation at Bitfinex, Wex and other unlicensed exchanges and the account/withdrawal locking at Bittrex and Poloniex, I'm only comfortable trading CFDs and derivatives right now.
1Fox (run by 1broker) launches on the 24th. I'm very excited about that. For now, I do all my trading on BitMEX. No KYC, no selfies. Just deposit BTC, trade with leverage, withdraw BTC.....