Quoted from Black_Knight:what you spent purchasing the goods, what you spent to package, ship, repair, etc. Basically anything you spent is deductible, even your monthly internet bill
I can not stress this enough:
SAVE ALL YOUR RECIEPTS.
If you get audited, they will demand to see them. If you do not have them, they will fine you and then add past-interest due on taxes owed, and depending on what they get you for, it could be a life-breaker.
While I'm not a seller on ebay, one of my jobs wants to convert me to a 1099, and I literally just spent earlier this week talking to a tax guy to figure out how to handle it, and this was one of his biggest sticking points.