I've had GREAT success on ebay! Old Navy, of mine, ALWAYS sells. CP is hit-and-miss. My rule is list it once as a buy-it-now, for 30 days. If unsold, then donate. Don't do the auction route - they only allow 7 day listings. It takes less time, and costs less to list, in lots. So if you have like 3-4 long sleeve shirts in one size, list them together. I do have to say I do better when I have multiple things listed rather than 1 or 2 at a time. As far as shipping, generally if I charge $3.95 for a kid's shirt, it cover the shipping (never more than like $3.42) and the envelope I buy. If it's 14-16 oz. charge $4.95 for a priority mail envelope (free @ post office). If it's 1-2 lbs it will usually cost $8.10 priority mail to ship. Over 2 lbs. ship via flat rate for $10.35. I've paid for MANY Disney souvy's with my ebay $$$!
HTH!