Bupp wrote:If I change cards in a deck, how to I move cards from between inventory and decks, and keep my counts accurate?
Does everything just show up in inventory by default?
Correct. "Inventory" is a list of every card you own, regardless of where it's located. You don't "move cards out of" your inventory and into decks. You simply list all cards -- both used in decks and currently unused -- in your Inventory, and then build your deck.
TL;DR -- Just list 37 Mountains in your Inventory, if that's the total amount that you have, and then put 8 in your Deck Idea. This means that 8 are used, 29 are unused or "extra". However, you will not able to see how many total you have in decks unless you're a Premium user.
For a non-premium user, all decks will just be "Deck Ideas" -- i.e. none of them "count against" your Inventory. In your Inventory view, the Deck Count column will just show the highest number you would need for any one of your deck ideas (so this number usually won't be higher than 4, obviously, except for basic lands and cards like Relentless Rats). So if one deck idea needs 2 Lightning Bolt and another deck needs 4, this column will show 4 (not 6, the total of the two decks).
A premium user can create "Built Decks", not just deck ideas. If a premium user goes into his settings and un-checks the option to "Share cards between decks", then the additional column in the Inventory for "built deck count" will show the total of that card in all built decks. In the above example, it would show 6, not 4.
The only shortcoming in this system is that, currently, there is no way to specify that a specific version of a card is used in a deck (e.g. your Signed NM Foil M10 Lightning Bolt). This is a feature that has been requested for a loooooong time. GOOD NEWS, though! It's on the horizon. Please see this thread in the Announcements forum and go test out the alpha version of this feature.
Note that it will still not enable you to "move" cards from your inventory to a deck, but it WILL count against a specific version of a card in your inventory. Note also that it will still require Premium, along with the Built Decks feature and the ability to choose to not share cards between decks.