So far I’ve come out net-positive each Halloween since I’ve started selling costumes. Take quality pictures the year you wear the costume (while it still fits growing kids and waistlines) and make sure to highlight any flaws. List the costume on Ebay a month or so before Halloween. As long as you can sell it for more than your shipping cost and 15% in ebay/paypal fees you’re saving money. Personally, I sew the costumes out of scraps for about $1 so I actually make money on Halloween.