I sold an expensive piece of furniture at auction this year. Is the net gain from that taxable income?

[Critter-3]

Yes, any personal property sold at a gain is reported on the Sch D/form 8949. 

[Opus 17]

Your gain is the difference between your cost basis and the selling price.  If you made this furniture piece, your cost basis is just the cost of materials, you can't include anything for your labor.  If this is a hobby, you can't subtract other expenses, but if this is a business, you have a lot of other responsibilities to think about.

If this was a gift, your basis is whatever the giver's basis was.  If it was inherited, your basis is the fair market value on the date the previous owner died.

Be aware that if you are unlucky enough to be audited, the IRS does not have to award any basis you can sufficiently prove.

This is a capital gains transaction, so you pay less tax than ordinary income as long as you owed the property at least one year.  Report it in the section for sales of stocks and other investments.