What this young person needs to do is to get his parents to help him document each item of furniture, including a brief description, how it was acquired by the parents (as a gift, purchase, or inheritance), the approximate date, and the approximate cost (if purchased or gifted) or the approximate value on the date of the previous owner's death (if inherited). A photo would also be very helpful.
This information represents the parents' cost basis in the items. When the parents give the items to their son, he also receives that cost basis. If he sells for less than the cost basis, he has a loss that is not deductible because these are personal items. If he sells for more than the cost basis, he has a taxable gain. The gross proceeds are not taxable, only the amount they are more than the cost basis (if this is even possible with used household items).
By getting the documentation now, he will know how to include the sales on his tax return (if any items are sold for more than their cost basis) and he will have the information to defend himself if the IRS tries to assess income tax based on the gross value of the sales.
