How do we know if a deck is going to truly hold it's value?
Not a dumb question at all, really. The answer is short, sweet and to the point from my experience.
We don't because it won't. It's a deck of, this is an important distinction, Modern Playing Cards. Collect because you love them, never for monetary value, gains or otherwise.
Are decks really valued on desirability & accessibility?
There is rare and there's
rare. With exception of very few decks, Modern playing cards never fit into the second category. There's just too many of them. It used to be that a print run of 5000 was limited. 2500 was "ZOMG these are going to be in demand!". Now we see so many Kickstarters with 1000 limited and several with 500 limited, what's rare anymore? I'll tell you. The 3 in the world of a certain vintage deck, that's what. Modern playing cards are everywhere.
Maybe what drives up the aftermarket price with modern playing cards from what I've noticed is popularity to fanboys in most cases. Watch their trends and you can pick up some stuff as an investment I guess.
I guess every deck is worth what your willing to pay for it!!
You are absolutely right! Every deck is what a person is willing to pay for it. I hope you're not looking at playing cards as though it could be something to invest in. That would be a horrible idea. Bonds are much better.