When I'm trading in D2 I often just need something small like a few runes and I hate finding what I need in somebodies For Trade thread only to see that they are offline. This would hopefully combat that a little bit.

Even if they were Last Seen: 2 seconds ago, they can afk for a longer time. And if they're offline they can come back the second after.

Nope.

Well it's going to be flawed but if your only argument against it is "people might be afk" then that isn't a very strong argument. Bear in mind this is would be optional, so at any moment you could choose to show thread from offline users.

I think it is a very good argument, and I agree.

Voted no

Then why bother having lists of online users at all?

That shouldn't happen if you check fresh threads.
or just make an ISO yourself.

Your idea doesn't work as well as you might think.