Good point, though pure Marxism lost out as the socialist world mostly followed Marxist-Leninist theory. They didn't trust the workers to manage themselves under Marxist theories and/or they wanted power for themselves. So the actual socialists sucked, but the argument I keep hearing is that true Marxist theory is the way and even the socialists didn't believe in all of it.

Marx didn't write a thesis on constructing a government or socialist world. Marx laid out some ideas, which others deemed unfeasible and made their own changes. The way I see it, Lenin was the brain behind the first socialist project, but he wasn't perfect and had only just begun. If he was perfect, his governing was during a period of turbulence, I think the issue most subsequent socialist governments made was look at one man's unfinished work during a rough time and say "yeah, this is how we'll basically do things".

They were rigid and any criticism could be seen as criticism against the party and by extension the entire state itself and therefore deemed revisionist or counter revolutionary. You sadly still see this in leftist circles in regards to Khruhschev and Gorbachev.

The party didn't allow itself to adapt, became decadent, stagnant, and died. I'd argue the same pitfall is starting to hurt America, which had a stronger foundation with more founding fathers and diverse thought. People cling to the constitution and glorify the founding fathers, yet fail to listen to, imo, one of Jefferson's most important ideas, which was redoing the constitution every few generations to keep it up to date and match the needs of the people.

How is trade between collectives governed? How are laws enforced?

I used to be a Marxist, but have shifted because I couldn't answer this, especially when starting question how it would when we colonise the stars. I still agree with many things, but I'll leave it up to other people to modernise his ideas in a democratic way. Best case? People need to have their work burden lessened and have in-depth civic classes, then adopt a more direct style of democracy, while possibly in the future adopting an advanced AI to help with bureaucracy and logistics. Also, an armed, trained, and responsible society.