I saw this thread and have been meaning to post for a few days...we had the same issue, a long saga for us. Ds4 or 5 at the time decided he liked the 'Yu Gi Oh' card game best, well he couldn't even read, so we waited to ok this until about 6 or 7. He's now 9, and it's gonna be another few years before other kids are thinking about that one...
Last year he just got so frustrated, he was crying, with not understanding why no one was interested in this super cool game. He said the only boy who did like it told him it wasn't even popular anymore (older/gt boy) , which REALLY broke his heart. I actually kept him home from school he was so messed up about this.
So, last year he got more into Pokemon, several of the boys in his dance class play, so that worked ok. There was 1 incident of trading, and hurt feelings, accusations of unfairness, it is just hard for ds to see it's JUST A CARD, JUST A GAME, etc. But he's getting better at it. He's had some fun with the video games, read some pokemon books and learned a lot of the trivia (great for the SAT's). Anyway, back to dance in another week so I know he's looking forward to playing with those guys, and it really can be a great game to play while waiting around for a practice, etc., to start.
One or two boys that we know well, whom we know to be fair guys works VERY well, and I generally have the 'no trading' rule in place. Once or twice since then ds has explained a pending trade to me, gotten my ok and gone ahead with a trade, which we find to be an ok way to go. Like Kriston said, making sure all sides are happy with it, and can even trade back within a day or two if they regret it. No playing for keeps, that's a good point, too.
The double whamming of losing the game plus losing the cards can just be too much.
