I think there's a way to stop that before it begins. Either by tracking IP addresses so they wouldn't be able to self trade, or if there's funky trades between 2 accounts flag it, and if it's come to light that that's what has been going on, not allow them to participate in DD (at least on the main account). May be a bit extreme, and difficult to do ... But I've only "collected" a few of my diamonds. There's def other cards I want, and would certainly trade to get and build a team with guys I want as opposed to it all being luck of the draw, or dropping an insane amount of cash for 1 card.
Trading baseball cards goes back generations. I think it's safe to say that most of us on the board did it when we were younger. It'd bring a different element to the game. The only thing you would have to worry about are the folks who think their cards are worth way more than they really are...
I can see the "not making them money" reason as being the biggest. But I truly think it would make what is already (imo) the best mode in the game even better. And that's no small task. Maybe they could charge a few bucks a month for a traders club? I would spend 5 bucks a month extra to be able to move my cards and trade them without limiting the trade amounts, or they could have it where you're only allotted 3,4, or 5 trades each month.... Or being included in only one version of the game. I got the digital deluxe version, so for $100.00, I think I (we) should get some leeway...
Who knows.... If enough ppl ask for something like that, I can see them doing it because they do enjoy pleasing their fans.