I finally did my taxes. TurboTax Premier did not guide me through what do about Form 1099-K. I searched TurboTax Help, and it returned this:
How do I report a 1099-K if I don't own a business?
TurboTax says to manually add the amount in Miscellaneous Income as hobby income, but don't declare expenses there. Then go into Other Miscellaneous Income and enter your expense (e.g. original purchase price) as negative income (up to the amount in the 1099-K). The result is that you only pay income taxes on the net gain (if any) when purchasing and then reselling an item.
My conclusion is that there is no specific place to enter 1099-K amounts on the tax form. However they must be accounted for as part of the total income, so manually enter it as miscellaneous income. Separately, subtract expenses by declaring negative income items. A bit convoluted, but it makes some sense. It would help a lot if TurboTax integrated this into their guided steps.
BTW, if you are a business, TurboTax basically says to ignore 1099-K. It is part of business income that should have been entered somewhere else on the tax form.