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Posted By: identify7

The number of active collectors = the number of cards listed for sale on ebay (150,000) x the number of purchases made by the average collector in a week (1) = 150,000*.
One percent of whom buy pre-1930 cards.

Edited to add: * assumes that the definition of active collector does not include those who primarilly or exclusively trade cards.

Further edited to add: I guess the relationship between these factors is one of division rather than multiplication.