Peace Sells...But Who's Buying? Lyrics Meaning

I think that the song has to do with experiencing apathy/nihilism/cynicism at the personal level (i.e. the metalhead stereotypes) when faced with awareness of the total saturation of global economies, media channels and political systems by the corporate military-industrial complex in all its forms and effects. Mustaine presented these ideas not just in the lyrics, but in the album artwork and the visuals for the song's music video.

"Peace sells, but who's buying?" - massive financial deals, the activities of powerful corporations and the influence-peddling of governments are motivated by violent economies (weapons, prisons, armed forces, destructive extraction of natural environments, etc). "Peace" is perfectly welcome to bring its wares and handicrafts to the free market, but alas, there are no [noteworthy] buyers. The market for war, destruction and control are what move the world system.

"If there's a new way, I'd be the first in line." - He doesn't have any ideas about how things should work, he's not a revolutionary. He's just acknowledging that the current system is rotten and he would get in line if there were an alternative.

"It's still 'we the people', right?" - this line is reminiscent of Eisenhower's warning about the military industrial complex taking over the US from behind the scenes. Mustaine grew up in the Cold War period and is aware of these processes/paranoias. I don't think it's a call to patriotism or constitutionalism per se, but more like an existential question about whether the political entity of the US is actually surviving the takeover by the military industrial complex.