So is the 7.62x39 in large surplus, even though it isn't a Nato round?
Regardless of the fact that its not a NATO round, it is still a military caliber and therefore you can find military surplus ammo for it. It's a little more difficult to find 7.62x39 in the US specifically than 5.56 nato and .308 nato, but that doesn't matter because Wolf ammo has just about the same quality 7.62X39 as mil-surp 7.62x39 at a cheaper price. So you might as well just buy wolf, or if you want much better performance you can get American factory 7.62x39 but at a higher price.
I've only bought mil-surp x39 ammo a few times and it was more expensive, yet less accurate and less reliabe in exchange for having a slightly higher velocity than Wolf/barnaul. The people who sell x39 surplus can exploit the catchy name of "milsurp ammo" and people think that it is higher quality and/or cheaper than regular factory ammo, which it isn't in my experience.
But its very rare to find x39 milsurp anyhow considering most countries who's military uses 7.62 are our enemy, or alteast an antagonist towards the US and therefore there is rarely a trade relationship between their markets and our markets.
So what are the advantages/disadvantages of 7.62 x 39?
Advantages: Cheap ammo, good short-range punch, usually coupled with relatively reliable rifles, and the ammo itself is usually reliable.
Disadvantages: Not very flat shooting/fast bullet drop, of poorer quality compared to milsurp NATO ammo, shortage of milsurp ammo (if you care), cheap ammo is non-reloadable, accurate loads are far more expensive than 5.56 and 7.62 NATO as well as their civilian designations of .223 remington and .308 winchester. Usually coupled with rifles that are reliable, yet inaccurate and with poor ergonomics.
The biggest thing that I don't like about the 7.62 is definitely its inability to shoot accurately at medium-long and long ranges. This may not be an issue as a self-defense rifle, but I much prefer the .223/5.56. I can hit targets accurately and consistently from 50-400 yds in rapid succession with my .223/5.56 rifles. This can't be done with a 7.62x39, simple as that.