Reporting 1099-K

I have a 1099-K from a where I buy and sell musical gear. The gross proceeds are about 5K but if you factor in the cost of what I sold we're taking a gain of about $300. Turbo tax does not offer the ablility in to include the total cost of these these items. Thi is not a business, only a hobby.

 

Right now my tax owed went up by $1300. How do I rectify this?