1) Most of us refer to this one...It's one of the cheapest (probabaly the cheapest) Money changer in Malaysia & rates much better than Banks.:
http://www.wawasanmoneychanger.com.my/exchangeRates.asp

& yes the rate changes everyday! but not realy by minutes, that's are for forex traders

2) Buy meaning they buy from you & sell meaning there are selling to you. So if u are changing RM1000 to USD at buy=3.595 & sell=3.615, u will be expecting to get 1000/3.615 =US$276.60. 

SO if u decided to change you mind & change back to RM, they willo buy back at RM3.595 =RM994.40 ..Meaning the diff of RM5.623 is their commission/margin...pretty small indeed...

So if u see the spread between buy/sell is huge, the money changer is reaping huge profit!!

3) Based on my limited knowledge (Correct me if I'm wrong), Forex trading is only in selected licensed houses (Normally banks) & sanction by Bank Negara (Not usre SC involve). So you buy/sell via them..The best we as public is buy/sell at money changers...haha..or some form of bank deposit in foreign currency...I bet those saving in Aussie/New Zealand $$ will be crying out now..