The 1099K's are for electronic paycard transactions, if you received a personal check, you would not receive an income document for that transaction. If it is something that is taxable, i.e. something you purchased with intent to resell for profit, then it would be reportable as miscellaneous income on your tax return, or the amount of profit on the item would be reported.
If you are selling an item that you owned for personal use and are just getting rid of things, that would not be a taxable transaction, unless you sold the item for more than you paid for it.