I have a very long list of decks I'd like to add to my collection, too. I like to keep my decks sealed. I will only open it if I have an extra deck. However, I usually buy only one deck because the money I could use to buy a second one usually ends up going towards another new deck I really really want, either on eBay or Kickstarter.
That being said, there are certain decks that I like so much that I can't help but purchase more than one despite knowing that I could use that money to add a different deck to my collection. These include the ultimate deck, fed 52 series, Aurum series, and Deco series.
Then there are those decks that cost so much, it makes me too reluctant to open it, even if I have two (or more). These decks are usually priced $100 and up. I don't collect decks as an investment. I haven't sold any decks and I don't have plans to sell them in the future, so my reluctance to open decks isn't due to a need to preserve their value for financial profit later, but I have a thing about preserving their value, regardless.