I have joined many car forums over the last 25 years or so. Too many to remember TBH. Lots of them had different 'rules' about for sale ads and everything else. Almost every one with heavily moderated rules are now dead, or just 2 old farts talking to each other about the 'good old days'.
All migrated to facebook pages, millions of them, where there are no rules but seem to just be constant arguing and bad feeling.
This is one of the last (from what I have seen) proper old school forums which is still busy and popular. There must be a good reason for this. I have my own ideas why, but they may not be correct.
I've been around forums since the early days of the internet both creating and partaking (used to work for a newspaper group) and I'd agree - too many rules generally cause more issues than they solve. In my experience, the communities can become clique's which then puts off new people from engaging (plenty will continue to join, fewer will feel comfortable posting and the spiral is difficult to pull out of) so I think openness, sensible feedback and basically talk to people how you would in the pub is the best guideline.
Often it's the stronger personalities that can kill a forum as they disproportionately distort the 'feel' of the place. It's a fine line between Troll and Serious Contributor but sometimes they just need reigning in (one to one rather than publicly) as it creates its own dynamic (i.e. in-fighting spirals). I see way too many times where a few select members just get into childish battles and it just stops new people engaging and the forum dies a slow death.
The VX220 forum was one of the best I've come across but had some technical issues and has now turned into the 'two old blokes talking about the old days' - it was THE place for VX owners back in the day. But the knowledge on there (on old threads) is incredible. Such a shame. I've indexed the info and passed it into AI so it's got less chance of being lost forever once the owner doesn't pay the £50 hosting fee...
Personally can't stand Facebook for this type of community. Too many groups, duplicate, thin posts, transient content, no consolidated knowledge base, etc - just does a different job to a forum for me.
FWIW, I quite like it here - seems fine to me. Everyone is friendly, plenty of comments, etc - I said 'hi' when I joined and got some friendly responses - a very good thing in my view. I'm not in all of the threads or areas but from what I see, no major problems so perhaps ride things out for a while and see if it settles? Personally don't see an issue with a dealer advertising a car - if it's a Z4 - someone on here might be interested. If they're posting Bitcoin BS then yep, get rid but if it's relevant and adds value, why not let them post? Let's not get too precious eh
Perhaps ask Dealers to mark the advert as such or the community members reply saying something like "for info, this isn't likely a forum members car" just for those newbies as Argyll Andy said. TBH, whenever I've seen a forum members car for sale there tends to be follow-up posts from other members anyway so as a new member, it was clear to me whether it was a dealer or not.