You should report every 1099-K that you receive because - as @ColeenD3 states above - the IRS is looking for all of the income that they have had reported to them to appear on your tax return.
In this instance you will report the income from one 1099-K as income and then the actual cost of the item that you sold as a negative.
Then, for the other 1099-K, you would enter the income as income and an equal expense ($1000 income and a $1000 expense) as "income entered twice". That way your actual loss matches what is shown on the return.
@Milad2608
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