How well this system works depends directly on the delta between how influential the platform is vs how influential the accused is.
As you mention, eBay for the longest time was one of the only/biggest way to sell things online, and reputation on there could make or break how successful your business would be. Similarly, if this forum is used by the majority of people doing business on the dark web, then obviously it isn't "moot" as not following the decision is equal to heavily handicapping your business..
On the other hand, if the group accused is large enough, like some of the $20M ones mentioned in the thread, I assume said groups could easily spin off their own community or still have business even if they were banned from this forum.
If they're known to scam customers, I doubt that their community will take off. People will research you when they offer such sums. You can, of course, start anew, but it will take a lot of work to get back to the reputation level where people are trading 7 digits with you. Getting banned from the forum is more of a gesture, the impact is a lot larger.
Of course, it's a different topic if the decision was against them for clearly unfair reasons.
I imagine its like ratings on ebay but much much higher stakes.