Yeah each item has its own ID, so they can trace it if it's dropped, destroyed w.e. However If you drop a poring card and somebody picks it up, I doubt the GM's will go chasing after it for you. =P
In terms of trades, I always take SS. The trade window open with both your offers filled and a clear sentence showing what the agreed trade is. I usually only do that with bigger trades or if i think somebody is dodgy. (selling a valuable item at a low price, etc)