The sale of tangible personal property is considered a capital gain or capital loss. You have a capital gain if you sold the property for more than the cost basis. The cost basis for inherited property is the fair market value on the date the previous owner passed away. Your capital gain would be taxable income, not the whole proceeds but the gain--the difference between selling price and cost basis. You may want to consult with an antique dealer or a published reference guide to see what the fair market value was on the date of the previous owner's death.
If you don't have a gain, you don't have to report the sale at all. If you have a loss (you sold for less than the fair market value on the date of the previous owner's death), you don't report it either because losses on the sale of personal property are not deductible.
In the unlikely event you are audited, the IRS could determine the entire proceeds are taxable if you can't produce a reasonable estimate of the cost basis. You should make a written record that describes the desk, how and when you acquired it, and how you estimated the cost basis.
