Combine the sheer volume of Series 2 printed with the fact that all the best cards in the set were in Series 1, and you have an excellent chance that most of those Mantle cards are still buried in otherwise unsellable boxes of Series 2.
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After posting that, I went to eBay to check the prices of Series 2, and everyone is clearly out of their mind. Everyone still seems to be trying to get $15+ per box and $250+ per case. At least Baseball Card Exchange is a bit more reasonable at $8 a box, but for some reason a case of 20 boxes is $200 (rather than the logical $160 based on per-box pricing). Either way, that stuff isn't getting opened any time soon unless someone is just looking to drop several hundred dollars with a minimal chance of return on that "investment".