The Importing Country places on an order on the Exporting Country/a Manufacturer in that Country.
The order is scrutinized by the Defense and Foreign Department of the Exporting Country and if found in order , then the order is through.
But nothing is perfect in the world.
We have Separatists like Batista, LTTE (till recently) and a host of local out fits who wage wars against States.
The Exporters are lured by the money and they sell.
And though there are no communist Regimes now, save China, USSR, North Korea and a few others.
They sell still for ideology.
Then we do have individuals, Loosely linked terrorists and Terrorist out fits.
The want to buy.
The Arms manufacturing countries lured by the money and taking into account their Geo- political calculations sell Arms to these people.
They can not sell these direct .
To fill in the void came Arms Dealers.