I can only see the "Average Deckbox Market Price" for cards in the trade window. Are the TCGPlayer-based prices disabled or is this a bug?

If this is disabled, is there a particular reason to do this? Can users have the option to enable it again?

This should answer your question better than a response I could give.

http://deckbox.org/forum/viewtopic.php?id=19404

That sucks. I guess TCGPlayer took action since Deckbox enabled card selling and thus became a "competitor"?