Create some kind of "Blacklist" and "Hall of Shame" for people that didn't finish a post exchange or just posted crappy things.
The karma system on the forum could also be used.
And in the directory you can also rate links. So positive rating if the admin of that forum did a good post exchange, negative if bad.
But it's indeed the post exchangers repsonsibilty to check the quality of the post exchanges or set additional rules for the exchange (eg at least x new topic, at least 3 sentences in a post,...) and to report a bad post exchange to me.
Then we will check it, and if needed remove him from the directory and put him on the blacklist/hall of shame.
When you submit a link to the directory it's possible to add a description, and even use bbc in that desciption. So settings these additional rules isn't a problem.
I'm starting to like this idea more and more...

[edit]
Just rememberd the trader mod for SMF:
http://custom.simplemachines.org/mods/index.php?mod=346Maybe we could use that one as well.
Opinions?