If you purchased the item, the cost basis would be what you paid for the item. If it was inherited, then the cost basis would be the fair market value on the day you received it.
Turbo tax does not have a source to figure the FMV on a particular item. The FMV at a time is generally the amount someone would pay for that item at that time. I would not say using zero as the COGS is the safest, because the profit from a "business" may create an earned income tax credit.
