In addition to regular ups and downs of the market place, there's the possibility of parallel universe type futures for marbles.
People do their retro thing, and it's possible that marbles could become a craze again if they're in the right place at the right time. Could become wildly popular for a couple of years and create a whole new group of people nostalgic for that time in their youth when certain marbles were must haves. Hey, it could happen.
Or by some quirk of fate or marketing some other toy could be picked for the retro craze, and that one could be the one which gets the nostalgic bump for the next generation or two.