I think that - if everyone wants it to be consistent - then they should post on the thread that they PM'd at time 'x' so other potential buyers are aware that they may have been pipped.
For example, there have been times where I have posted in a thread 5 minutes after something went up for sale to then have the person after me post "PM'd" a minute later. Now, I may have posted in the thread a minute before the other person but if they PM'd 2 minutes after it was initially listed, then it is them that should have first refusal (IMO, of course).
Then, the seller can view when the PM was sent and when the thread was responded to and act from there based upon who got there first.