Military surplus vehicles are usually auctioned off. These include land and air vehicles like the Dakota aircrafts that can be used for private flying or for transportation or airplanes like the P -51. However, a license is required to sell or export these vehicles .If it is known that these vehicles can transport nuclear weapons or parts thereof then their sale is prohibited.

However, the following usually are prohibited from sale.
Tanks and self propelled guns.
Half Tracks
Vehicles fitted with mountings for arms.
Also military surplus vehicles may not be sold to certain countries like Angola, Rwanda, and Iraq Etc.

The way to judge whether a vehicle is just out of service is to see the condition, the paint, whether the insignia has been blacked out recently. Avoid vehicles that look like that they have been standing very long in one place, also if the parts look like they are replacement parts. Flat tyres and oil slicks also indicate poor health for the vehicles. The vehicles that are scrapped are those that do not meet the safety standards. The process is to participate in sealed-bid and auction-surplus sales.

The reject vehicles are first offered to the military wings, related agencies and government organizations and charitable organizations for reutilization. Only the ones that are left are offered to the general public. The process is actually quite simple and does not even involve much paper work. The storage, cleaning, upkeep and disposal are all handled by the military.

In India, Ex-Servicemen / widows of Defence personnel who died while in service  are eligible to apply for allotment of an Army Surplus phased out Class V-B vehicles.

Ministry of Defence,Government of India

It is important to know the condition of the military surplus vehicle that one plans to buy as physical inspection at close quarters is not always possible and one is not allowed to take the vehicle for a drive. Therefore always inspect more vehicles of the type one wishes to purchase and check for the condition of the parts. Reliance on ones practical knowledge in these matters is recommended.
There are agencies available that supply the trends in bidding of military auction vehicles for a fee