|There's more than one way to do things|
If there's no clear publisher, you should assume there's a copyright holder. Why? Because that's the default. Anything that is created is copyrighted - unless it's clearly marked otherwise.
Threads in a bulletin boards are NOT in the public domain (I do not know what "common domain" is) by default. Someone wrote them, so someone does have the copyright. That it may be hard for you to trace down the author doesn't mean the author doesn't have rights. Note also that whether or not something is copyrighted has nothing to do with commerciality. Nor does it mean that if you are "non-commercial" you suddenly have more rights. It might make a differences for the amount of damages you have to pay though if you'd lose a copyright suit though.
As for republishing the same data with a different interface, there have been court cases against websites doing exactly that. I haven't heard of cases where the republisher won such cases, but I know of cases where the republisher lost.
Note also that continuing to republish even after the original documents have been removed goes even a step further.
Specially in cases where you cannot contact the original author, it is questionable that you should keep republishing the documents after they have disappeared. You do not know the reason why the documents are no longer available. Perhaps the information is stale, or outdated. Or perhaps they contain errors. Or it would actually be illegal to still publish that data.
I see many reasons not to republish, both legally and morally. I don't see many reasons to do - you don't have an absolute right to the fruits of someone elses labour (just like noone has the rights to your labour (unless you have a contract)).