I agree, this goes both ways.
I had the same circumstances happen to me, I posted "I will take it PM sent" got a reply "OK" next thing I know he turned around and sold it out from under me between PM's exchanging payment details, this was all within an 1/2 hour. So the other side of this, if you're selling something and you have a committed buyer stating "I will take it" then sell it to that person and not the next guy. Also you should read back on the post and see who posted first.