I think Leon's approach was best, to estimate based on a known fixed number like total population and then take a percentage of that based on a personal experience/common sense guestimate of how many people out of X that you pass on the street might be a card collector.
Another problem with guessing based on a percentage of total sales, besides there being so many sources of sales, with private companies not necessarily publishing that information, is the problem of figuring an average spend per collector per year---given that number could be wildly compromised by dealers/speculators in the hobby who aren't collecting per se but bidding against everyone for items simply to try and flip them. This could variously inflate or deflate the guestimate of collectors depending on how you look at it.
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