I'd suppose each collector's definition of mainstream may differ. My own initial inquiry was in reference to a card that is part of a nationally distributed, complete, MLB set-- one that included the MLB players of that year, and was available in traditional packs for kids. For example, the "48" Leaf set.
Others may feel nationally distributed bakery cards are sufficiently mainstream. I'm curious to know the answer for each definition of mainstream though. It's all interesting to me.
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