I think these are both good points, but I think a "solution" to either one would be more complicated than the problem is worth.
There's the problem of people necro'ing threads all over the site, and we just lock the thread whenever it happens.
Ketara updates the reputable trader's list pretty frequently. It
is a lot of work (thanks, Ketara

) but I also don't see it as being ultra-time-dependent. Unless it's someone's first trade, most posts just add to their "total" of successful trades, and doesn't affect their standing all that much if someone looks for their name in that thread.
Just my $0.02