Off the top of my head:

Things vary with political climate, during "Clinton Years", it required loud public outrage to stop the government from paying for mass destruction of 'obsolete' M1 Garands.

Basically, since 1903, 'civilian sales' were allowed through Director of Civilian Marksmanship, now called CMP (Civilian Marksmanship Program). There has always been criteria to 'screen' buyers. The program was founded to encourage and develop marksmanship in the U. S. civilian population.

From 1903 to 1960's, NRA membership was a requirement.

When I bought my DCM M1 Garand in the 1980's (for approximately $121), an FBI background check, Proof of Citizenship, proof of competitive shooting involvement, and membership in a State Shooting Association or DCM Affiliated Club was required. (I waited almost two years for my order to be processed). BTW - You sent a Postal Money Order with the Application, you were happy and patiently waited, with excitement, for the Mailman to bring you a rifle!

Things & system have changed much with creation of CMP.

Search CMP to find current purchase requirements and items available. Sales are now done through North and South CMP 'Stores' at Camp Perry and Anniston.

CMP has become very cognizant of Collector Values. Current application process, and money you deposit, to get into the 'Lottery', for a chance to buy a 'released' G.I. .45acp Model 1911 Pistol, reflect this.

In relatively recent memory the CMP sold: 'Greek' 1903 Springfield returned rifles, 'Danish' returned M1 Garands, Bavarian Forestry Dept. returned M1 carbines, and VFW returned Krags.
Some collector/enthusiast/shooters probably did well purchasing this stuff on lucky trips to the CMP Store! Some of this stuff is now getting 're-sold'.

Back in the old days, a lot of obsolete stuff was probably sold in large lots at official auctions to firms like Bannerman's.