Why does it matter? If you get a pm before someone posts on the thread the pm has dibs. That's how dibs works, period. If you put an ad in the paper and someone called you the day of publication and someone else placed a response in the paper. Who would you sell to first? Another example: if someone came directly to you and another person called you after that, who would you sell to?
I understand that if its posted on the thread, everyone can see it and if its pm'd you have to trust the seller. But if you can't trust the seller on dibs, why would you think you can trust what he's selling?
Just my thoughts, but what do I know, I'm not evens paid member yet.