Do you mean buying additional permanent in-game cards for crowns? That could be rather unbalancing.
Or do you mean buying some real-life cards that are equivalent to the in-game ones, so that you can play Wizard101 card battles the way you might play Uno, and not affect what happens in-game? The problem with that is that the computations could get messy enough to slow the game down a fair bit.
A trading card game at a table would necessarily either be greatly simplified from the game or else require you to do rather complicated calculations that would slow things down too much.
Suppose, for example, that you use helephant. You have a fireblade and weakness on you, with fireblade cast before weakness. You have +4% fire accuracy from gear, and +7% fire damage. Your opponent has 8% fire resistance. Your opponent has a fire trap, the fire part of an elemental trap, and the fire part of a volcanic shield, cast in that order. Name the five possible damage values that the helephant could do. See the problem?
Personally I don't think if they ever made cards they would do that. + It would be a simple not 700-800 damage it would simply be 800 damage. The blade should take of 25- damage which makes 775. It wouldn't be a lot easy if we forget this and just make ninja pig plush toys ! they would be so cool. I WOULD SO BUY ONE<3
If blades, traps, and shields were additive rather than multiplicative, you'd lose most of the depth of the combat.
But that's kind of my point. In order to make the computations simple enough that you could do them by hand, they'd have to greatly simplify the combat system. That would likely kill what makes it interesting.