Can you count your sold furniture as income? Do you have to show how much it cost originally? Do you have to prove you bought it originally?

My friend's parents are moving and he is helping them by selling their furniture. His parents bought the furniture, but because he's doing all of the leg work, he'll take the funds from the sales. In this case, does he need to report the funds as income? His parents don't have the receipts for the original furniture cost.