The illegal gun trade is actually far less formal and substantiated of a thing than it might sound. Most illegal guns actually do not begin their existence that way; they are originally purchased as perfectly legal guns, at gun shops. Gun crime, as a result, is actually originated at the gun stores from which the guns are obtained.
While the gun stores cannot be held accountable, as they likely sold the guns under perfectly legal conditions, the fact is that criminals with an intent to perpetrate gun violence have easily found a way around America's gun control legal system, and are able to use it to their own advantage. Gun crime is easy to commit when one can obtain a gun in one state, and move it to the other, without much difficulty.
The illegal gun trade often takes its form in robbery and casual exchanges of weapons. Young criminals can steal guns from either homes that have guns, or from gun stores themselves, and thereby arm themselves. Also, many who have caused gun violence received their first gun from a family member, or a fellow gang member, instead of buying it legally. So the illegal gun trade often has this element of informality that makes it all the harder to combat.
Gun violence does, technically, include the misuse of legal guns as part of its purview, but a great deal of gun violence is caused by guns diverted through the illegal gun trade.
This is especially because in many of the areas of the country in which conditions can lead to gun violence, state legislatures have specifically set out to prevent such gun violence by creating stronger restrictions on the purchase of guns. But the illegal gun trade allows for guns to easily pass from state to state, allowing purchasers to buy guns in one state with lax restrictions, and then transport them and distribute them illegally into another state.
A recent report on Maryland, for instance, found that nearly half the guns used in gun crimes in Maryland were actually illegally imported into Maryland from the other states. Maryland's own highly restrictive gun laws do not stop the illegal gun trade from bringing more weapons into Maryland.
Gun crime often increases even in the face of stricter gun regulations, specifically because of the power of the illegal gun trade. With guns being so easily accessible, thanks to the facility with which an illegal gun trader can get guns from one state to another, it is no wonder that gun violence continues to be a problem, even in those sections of the country with the most strident gun laws. To a certain extent, even though different parts of the country require different laws, the illegal gun trade can only be truly affected if gun laws become more uniform throughout the country.
Treating such laws as separate things for each state only results in the gun crime being able to grow through the illegal gun trade; instead, accepting that the states are interconnected could be one of the best ways to make a huge dent in gun violence.