1. The IRS requires that a PSE (payment settlement entity, such as Offer Up or Facebook Marketplace) report the gross amount received on your behalf. Gross amount is further defined by the IRS as "total dollar amount without regard to any adjustments for credits, ...discount amounts, fees, ...or any other amounts". The answer to your first question is that the 1099-K will have gross sale amounts.
2. If you are in business as a reseller and file Schedule C to report your income and expenses, you must report all income you receive, even if you don't receive a 1099-K because the amount is less than $600. Your income on schedule C is the amount of revenue received and one of the expenses you have would be inventory sold. The difference between revenue received and inventory costs should be positive in most, but not all instances. The answer to your second question is don't ignore revenue received and also include all of your inventory expenses.
3. Filing nonresident state income tax returns would depend on the rules of that state. The answer to your third question is generally you only pay state income tax to those states where you store and from where you ship your inventory.
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