Jimmysmith,

That is a little sketch.  If I do a .5btc or a 100 btc trade all through PM or email or skype or chat I'll return to Bitcointalk.org (or OTC)  and both traders will +1 or somehow each trader will publicly confirm that the trade took place.  

If you did not know about that you should start.  If you did know about that and you don't publicly memorialize then trade the like I said, Its Sketch and should be concluded that the did not happen.