You know what really tanks a "collectibles" value, people going out of their way to preserve them. The artificial collectible market is never going to be anything like the accidental one. The odd object that used to be commonplace, and hence hold nostalgia for the current "in" generation (the middle aged ones with money and means), but was actually almost always used, hence mint condition items are rare, is where the value is going to be. Think baseball cards in America (where the market really was booming a decade or two back, but has since gone into decline), the point the market will peak tends to be when the generation that grew up with them has money, time and the desire to get some of their childhood back.
Amiibos are likely going to be very mass produced, and this first generation is going to be restocked many times over. The time to sell for the most value is either going to be now when there is limited stock (for ones like Marth) or in decades if you happen to keep one mint in box and hope to cash in on that sweet sweet nostalgia, if there is any, it might end up a commercial flop (though that could help the potential future value).
Long story short, unless you're going for the quick resell (while stocks or low) or are planning to take a gamble on the long game (and sell when nostalgia and low supply kick in, which may never happen if everyone does the same), just open them and enjoy them.
