I'm of the opinion that what you describe is a gift situation, not compensation, particularly since the parties are related. The parents have given your friend the furniture. The cost basis in a gift is the giver's basis. It's safe to assume used furniture has been sold for less than the original cost, so there is no gain (profit) on the sale. So, none of the sale amount is taxable income to your friend.
Technically, if the used furniture is worth more than $15,000, the parents would be required to file a gift tax return.
"Gift Tax" is somewhat of a misnomer. Even though a gift tax return may be required, very few people ever actually pay federal gift tax. The purpose of the gift tax return is usually only to document a reduction in the allowable estate tax exemption.
See https://turbotax.intuit.com/tax-tools/tax-tips/Tax-Planning-and-Checklists/The-Gift-Tax-Made-Simple/...
