From the trading post rules, which can be found in sticky threads at the top of each of the three sub sections of the trading post:
"Buyers,
First "I'll take it" post in the ad takes precedence over other responses. No soliciting counter offers, This site IS NOT AN AUCTION SITE! And seeking counter offers is starting an auction!"
No one but the recipient of a PM can see the PM, posting "I'll take it" in the ad lets EVERYONE know that "dibs" have been called on an item.