I have been selling children's clothing on Ebay for over 6 years and have been a PowerSeller for nearly 3 of those years.  Through tried and tested methods I have learned which name brands sell best when it comes to children's apparel. 

Gymboree used to be a top selling brand but Ebay is saturated with Gymbo now.  Unless I find it dirt cheap I don't purchase NWT items from the retailer itself for resale anymore.  Occasionally I find pieces at thrift stores or consigment shops which I know will sell and thus purchase those to fill my store.

Baby Gap and Gap now outperform Gymboree when it comes to Ebay.  Listing complete outfits is a bonus in terms of profit, yet I do find that single items tend to do well also.  Any item with the word "Gap" on it always sells.

Boutique brands such as Hanna Andersson, Baby Lulu, Oilily, Greggy Girl, etc...are always top sellers.  Hanna Andersson playdresses with matching leggings are hot items as well as their outdoor wear!  It's not out of the question to make back every penny you've put into dressing your own child with these names. 

Save Old Navy and any Sears, Target and Walmart brands for fillers in large lots.  You're not likely to make big bucks in single auctions with these brands.