A forum I frequent has an etiquette to post an offer or intent in the thread for all to see to avoid confusion. You can still keep your offer between the two parties if required.
That said you still have the odd post with the same date / time stamp but rarely happens.
From my point of view; if the PM bid was earlier and an offer was was made within the PM @ or above your requested price then that would be the winner. If it was just a "I'm interested PM" with the option to haggle then the forum post "with a confirmed offer" would win.
If they both expressed interest but no offer - No idea... first gets to buy it at said $ before you offer it to the other?
In the end your sale, your choice I guess.