This would be obviously very useful and probably very easy to code as well, though the loading time might get longer.
This was my idea as well, and instead of starting an entirely new thread on the idea, I'll semi-hijack this one x,x
Now that we have the import/export function, I see this as being pretty easy to do. When you import/export in the normal game, it checks to see if you have those cards or not. I suggest that this check be removed in the trainer. That way if you are copying deck codes (with sharing decks, as Kael is suggesting,) you can import someone's deck without having to buy all the cards for it.
It's not a perfect solution, since if you are making your own decks, you still have to buy all of your own cards. I just thought this would be a step in the right direction, that would be simple to implement, and would make using deck codes a little more popular