The following reasons are why I don't use Deckbox for serious trading or management:
The Dealbreakers:
1. The pricing per set seems to be based on the latest printing of a card, or seemingly - sometimes, a random one. I have 56 Alpha/Beta Islands in my collection. They are worth a slight bit more than what's coming up. About 800% of what's actually showing up.
2. Foil pricing. Add this. It's not really a request. More of a command. It's ridiculous that there aren't foil prices. Now, is this a byproduct of TCG's API? Maybe. In that case, stop using them. Otherwise, at the very least pull prices for foils from somewhere else like Ark42 or something: http://ark42.com/mtg/pricelist.php?f=1
The Quality of Life stuff:
1. Unfortunately, TCG does precisely nothing with foreign cards. This isn't really obnoxious for new stuff as people can just look at StarCity, but for cases where the foreign cards outnumbered English cards - specifically P3K and Legends - the pricing here is obnoxious. I have an English and Italian Tabernacle in my collection. I'm willing to trade either under the right conditions, but am not willing to put either up for trade as the pricing is dead wrong due to the difference in languages.
2. I'd like to show my tradelist without showing my collection. Why can't I seem to do this. If I can, where is this option?
There's more, but these are the big ones.