I sold an item, that I personally owned, to a business for more than the $600 1099 reporting threshold. This sale was not part of any money making venture. I sold it for less than my basis in said item, so the money wouldn't be taxable, though the business would have no way to know that. When I eventually receive a 1099 for this, how do I report my basis to prevent the money being reported as taxable?
You will probably not receive a 1099 from the business for a simple sale of an item. 1099s are for when businesses pay an individual for work performed, or similar situations. (I receive a 1099 from the Amazon Vine program to recognize the value of the "free" items they send me to review.) You won't have to report any income from the sell since you sold it at a loss.
If you have documentation of how much you paid for the item, and when; and documentation of how much you sold it to the company for, just retain that information for 3 years; unless you had a lot of transactions (IE you are in business of buying / selling items), you don't need to report the proceeds as income.
If you bought a $1,200 riding lawn mower 3 years ago and sold it for $1,000, that's a loss.