Get your taxes done using TurboTax

For selling price do I list the cost that the buyer paid, which includes sales tax, or do I list my net earnings on the sale? Also, some of the music gear that I sold I got through trades for other music gear, but I didn't keep track of which item I traded away and the price of the item. In that case do I put the cost as $0 and I will be taxed for the net earnings I got from the sale?