you can send the gang leader a pm ((say its a letter in double brackets)) saying that you will supply their gang with [materials,drugs or weapons] for a price. normally you will follow market price but if you want the gang to be loyal you should give some discounts. Personally I think mat running is the best job. (((((((I would not suggest a forum topic because of MG issues)))))))
An example of a letter would be :
I've heard that you and your gang are in need of some [materials,drugs or weapons]. I am willing to sell you a large amount of [materials,drugs or weapons] for the market rate which is [price here]. Normally, i only sell at the market rate but if you and your gang become regular buyers I will decrease the price from [price here] to [price here]. This will benefit you and your gang because you will be able to [do something here] cheaper.
Please reply soon.. I think these deals will be beneficial for the both of us.
Once you are in the deal try and get a body guard because you may get cheated. Observe your surroundings for police and other gang members before you make the deal. Always remember not to give too much away in discounts as this could lower your profit.
remember to RP nicely ;D