Law enforcement agencies
Law enforcement agencies can sometimes be a source for some kinds of firearms. Different agencies vary in policy and practice. Some agencies sell their issue weapons every five years, some more frequently and others less frequently. Smaller agencies commonly have a slightly more frequent turnover of their arms inventory. The older guns that are being phased out are usually first offered to the officers and employees for private purchase. These people usually own plenty of guns and do not buy all of the guns that are made available to them by the department. Several of the weapons are then auctioned off or sold all at once to a third party. The third party is usually a wholesaler or supplier. By the time the guns make it to these people, they just go through regular channels to retail dealers as used guns.
The savvy survivalist, however can work the network of privileged buyers. One way you can do this is to network with any friends or relatives who may be employed by a law enforcement agency. There is rarely any legal condition placed on the officer regarding how long he or she keeps the weapon before reselling or giving it to another person. The situation is usually not questioned unless the guns end up in some sort of crime or are circulated in large numbers to commercial channels by an officer (or deputy) who is selling guns as a side business. The infrequency of the availability of guns through police departments and officers makes this an unreliable regular source for high quality used weapons at low prices, but it can be a good occasional windfall for someone who keeps the lines open for it.
Volume pricing is usually only open to licensed dealers buying at auction, otherwise it is simply an issue of an individual buying a small number of guns for personal use. One rare exception would be an individual buying weapons for use be a security company of the security arm of some organization. In rare cases, like scenario two situations, agencies may be amiable to making surplus weapons available to neighborhood watch and other auxiliary personnel.