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fkwRight now there is roughly 6500 auctions in the Pre1930, and 8000 in the 1930-39.  I can remember when there was around 1000 auction in either category, and a long time ago under 1000 total in the old pre1950 category.   I think more people sell their own cards on internet (eBay/websites), and dont sell to dealers, or at shows anymore.  Shows have died down alot since the Internet took off in last 10 years.
I dont think they are drying up, they are out there on internet, but no longer at shows, card stores, or in print ads.