I see a lot of threads for trades with either:
1. I have X, Y & Z to trade PM me with offers.
2. I need X, PM me with what you need for it.
My suggestion is that the OP closes the thread a soon as they have posted it. Therefore you don't get all the lame 'Good Luck' or 'Nice offer' or even 'PM sent' posts which are just pointless.
If you need to make a trade with that person then just PM them as they requested. If you want to know that they got (and read your PM) just tick the 'Request a read receipt for this message' box before you send it. So long as they confirm this of course.
If you don't know how to cloase a thread here's how:
You can also re-open the thread to bump it (I think, I'm not to sure about the doublepost automerge thing - Mod/Admin?).
I know this won't work universally, but hopefully for those just wanting to deal in PMs it should help.
Let me know what you guys/gals think...
EDIT: As I was about to hit 'Submit New Thread' button I noticed there is a tick box to 'Close this thread after you submit your message' lol