Basically there are three types of arms trafficking, which is also known as gunrunning. First of all there are public deals between two governments. These often focus on major arms like battle ships or tanks. The biggest exporter are the United States of America (31% of global share) followed by Russia (27% of global share) followed by China (5% of global share). The biggest importer of those arms are India (15% of global share) followed by Saudi Arabia (5% of global share) followed by China (5% of global share). 
In the annual arms trafficking report by the Small Arms Survey only three tier 1 exporters are listed: the United States, Italy and Brazil. All countries received a transparency rating as well. This rating identifies Germany, Switzerland, the Netherlands, and Serbia as the most transparent top and major small arms exporters. Iran, Israel, North Korea, Saudi Arabia, and the United Arab Emirates (UAE) are the least transparent major exporters.
The second type of gunrunning is when governments sell weapons to local militias. Some of these deals are unofficial while some are official. Governments have different reasons to do those kinds of deals, but often governments try to reach their targets while avoiding political uproar.
The third form of arms trafficking is when a single person or a group starts their own weapon selling business. In this case the major aim is to make money. These persons often do not care about who they are selling the weapons to. These deals often focus on small arms like machine guns or assault rifles or on light weapons like grenade launchers or portable anti-tank guns. To name a few famous ones: Monzer al-Kassar (*1945, sentenced to thirty years in prison in 2007), Viktor Bout (*1967, sentenced to 25 years in prison in 2012) and Adnan Khashoggi (*1935, died in June 2017).
In 2014, according to the "Trade Update 2017: Out of the Shadows", the international small arms trade was worth at least USD 6 billion. Ammunition accounted for 38 per cent of global transfers.
As reported by Amnesty International around 508.000 people get killed by guns each year, most of them in non-conflict settings. 
Even though Iran, North Korea, and the UAE rarely record arms exports worth USD 10 million or more in Comtrade (pseudonym for United Nations International Trade Statistics Database), research shows that they are major arms exporters, too. It is even more difficult to determine the status of Saudi Arabia, which seems to be a significant re-exporter of small arms.