Please note: A gun "purchased on the street" is not illegal.
Buying and selling a gun to/from a private party is not necessarilly an illegal transaction.
If I have a gun, and want to sell it to you, it is perfectly legal to do so so long as I do not know that you are going to use the gun to commit a crime.
If you do commit a crime, I may be liable for damages (both civil and criminal) if it can be established that I knew or should have know that selling the gun to you would create a public menace.