@ Manmbao,
I have had previous experience in the gas selling biz.so a few tips....before you open up the first hurdle you will go through is getting a city council license. They will say they have to come and inspect the shop and if its close to a residential area you may be forced to bribe them...they know how hard it is for you to operate without one.
Secondly, after getting the license they will ask you to go and get a fire extinguisher permit and from my experience at the fire station, its not easy if you tell them you operate a gas biz.
When buying the cylinders try and source them from the individual gas wholesalers IE not the big companies where they may give you a good rate of between 4-6k per 13kg and 3 for 6kg cylinder.
The most important of all check out the legislation regarding selling of fuel and petroleum products enacted in 2008/9 if am not wrong and it will guide you...google to see the laws. Good luck!!
Common sense is the most evenly distributed quantity in the world. Everyone thinks he has enough.