Trading Toys

This is an aspect to parenting I didn't see coming.  My 6 year old son and his classmates trading toys out on the school yard.  You see, we don't let him bring toys to school.  Then one day as we were walking home I knew something was up.  It was the little secret smile he had on his face, combined with his hand in his pocket.
"What's in your pocket?" I asked.
"You know there's something in my pocket?"  He's amazed at my Supermom capabilities.
"Yep"
"This, J. gave it to me" and he holds up a Bakugan. 
"Mmmhmmm"
"And I'm going to give him my Obi-Wan ship because I don't play with it that much anymore." (the Obi-Wan ship is a small McDonald's toy, not a big deal)
"Are you sure you don't want it anymore?"
"yep"
"and it's okay with J's mom that he give this Bakugan to you?"
"I think so"
"well, if you are sure, then I guess it's okay"
And it was for about a week or so. Then he decided he wanted to trade back. J happily accepted the Bakugan, but can't seem to find the Obi-Wan ship. Oh well, I chalk it down to learning experience for my son. 
So where do you toe the line?  I didn't mind the ship because it was a 'free' toy.  But what about other things?  I'm really not big on this whole trading business.  Is is just a way to learn about the real world?  Or is it a way for your kids to get taken advantage of?  From teaching I know the problems trading can create.  Friendship fights, theft, distraction from their school work are just a few reasons why schools have banned popular trading items (Pokemon cards and Silly Bands are some recent examples of banned at school items).  Most schools discourage bringing toys of any kind as they are susceptible to being broken, lost or stolen.  For now, I've left it with my son that his toys are for sharing, but not trading.  He seems to agree that this is a good idea...for now,