I don't have strong opinions on the discussion threads; I slightly prefer quoting to tacking on the end of an old thread, but both work.
However the transactional ones really have no reason to stay open. Even on the off chance that an old listing or request is still viable, PMing the author is what you want to do anyway.
Especially something like "WTB: Made-up-word brandable .com, budget $$$$". Everyone knows how to use a registrar at some point has tried to find that one buyer who wants their made-up, pronounceable, brandable, aged, slightly shitty domain, not yet realizing that it's probably them.
Ah - I just described a very compelling reason to keep that thread open didn't I. Endless supply of new domainers
