Ramifications of Illegal Acquisition
Obtaining weapons by illegal means does not guarantee that they will be secret, but they can be deniable. Remember that there will still be witnesses to the transaction along with fingerprints and other physical evidence. Witnesses can talk and physical evidence can be found, but it usually takes fairly active efforts to get these. Unfortunately, these efforts are about unpredictable. You never know when a friend or business associate might get busted or you might simply get pulled over and searched. Even a burglar can turn you in. If illegally obtained weapons are stolen from you in scenario zero or one situations, you can expect no official help from the authorities.
If you use an illegally acquired gun in what is otherwise legitimate self defense, it can poison your case badly if it ends up in court. Here you have to make some serious choices as to what guns you chose to have available for defense if self defense is legal where you are and you expect to make a reasonable defense in a court of law. Same goes for guns that may have been legal when you bought them but later became illegal. Remember that In the U.S. and Canada, 60% of all homicides are "cleared" (meaning that the killer is identified) by law enforcement. Of those, only those that are clearly self defense don't get to court. Of those who are charged, about 20% beat the case in court; usually for lack of evidence. Of those convicted, about 1.5% are innocent and wrongly convicted. The statistic that nobody wants to talk about is the number of homicides that result in a criminal conviction in one jurisdiction but are under identical circumstances and are deemed justifiable within a different court. This inconsistency is often glaringly obvious when the shooter is a government functionary like a law enforcement officer.
The advantage of black market weapons is that once you dispose of them, it is extremely difficult for anyone to prove you owned them. Consider that 40% of homicides go unsolved, chances are that most done with a firearm were done with an illegally gotten firearm. A secret, illegal weapon is commonly the ticket to deniability. A secret, unregistered arsenal may in fact be more feared than a legal one for these very reasons. Realize that while it is possible to have both legal and illegal guns, they should never be confused with each other and extensive security measures have to be used to conceal illegal weapons. While you may see an advantage in "letting people know" you may have illegal weapons, the actual physical security of them remains extremely important.
Note there are some moral and ethical considerations on what constitutes "illegal" when compared to what is universally wrong. It is common for authorities to tolerate people in possession of contraband items that were obtained through clandestine manufacture, smuggling or simply not relinquished when a law changed, but tolerance of people trafficking in stolen property is another matter. Even most militias and vigilante groups will take a very negative view of an individual who would possess stolen property. This intolerance will shift according to local laws and social norms, but it remains strongly in effect when a person is granted legal access to the same weapons but still chooses to obtain stolen property.
Automated intelligence gathering computer systems cannot reliably track you with illegally obtained weapons as long as they and you never come into contact with the system at the same time. If properly hidden, these guns can be accessible but deniable. You can also gain this advantage with the simple private purchase of firearms in jurisdictions and circumstances where permanent records will not be maintained.
Stolen guns can often be cheaper than their legally obtained counterparts, but in low to moderate overall threat level scenarios, you are accepting what is usually an unjustifiably legal risk in having them around. Not to mention other risks associated with the chance encounter with the weapon's rightful owner. I could assure anyone, that an person who is in possession of firearms that had previously been stolen for me would be in an extreme high risk predicament.
Guns obtained on the "gray market" often have the advantages of both worlds, especially in states and jurisdictions that allow for unregistered ownership of firearms. These often trade anywhere from below their actual value to a moderate premium depending on the market and individual circumstances. Unregistered guns are, however not the ultimate ticket out of trouble. If you cannot prove legitimate ownership of the guns, unscrupulous individuals may be able to falsify reports to indicate they have been stolen. Having examined this carefully, the best way to deal with this is to retain records of ownership, preferably including a bill of sale of some sort from the previous owner and if possible, a written statement from a law enforcement agency that the gun had been checked and was determined not to be stolen. These statements can be difficult to obtain, but it is possible. Note that in states and jurisdictions that do not have mandatory gun registration, you can obtained notarized letters and statements asserting ownership of certain property and retain these in private possession until such need might arise. If something were to happen in which those records might be incriminating, you may be up for additional charges if you don't relinquish such records, but likewise, you are still not obligated to furnish a hostile prosecution with evidence useful to convict you.