> buyers decide the prices they'll buy at.
Not if they're index funds. They buy at the price it is, until they've satisfied their holdings represent the appropriate share of the market. Which, pre-IPO and early-days-after-IPO, is likely to not be accurate to the long-term price.
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This, of course, is why it's wise to wait a while after an IPO for the market to figure out how it values a new stock before it joins the index.