There is no link between guns purchased at gun shows and gun violence according this extensive study done on the matter in 2008 by the University of Michigan:
http://closup.umich.edu/res...
Per the study:
If anything, we find evidence of a modest decline in the number of homicides following the average gun show in Texas, though our aggregate implied effects amount to just one percent of all homicides in the state of Texas. Taken together, our results suggest that gun shows do not increase the number of homicides or suicides and that the absence of gun show regulations does not increase the number of gun-related deaths as proponents of these regulations suggest- Mark Duggan
Whether a gun purchased legally or illegally is irrelevant to gun violence except in proof that some people will buy guns illegally; it is a Mobius argument for the point of determining gun violence and crime.
According to the FBI, roughly 16,272 murders were committed in the United States during 2008. Of these, about 10,886 or 67% were committed with firearms. Out of those, 672 (5 to 6 %) were linked to guns purchased at gun shows in 'potentially' illegal transactions.
