When kids grow as fast as they do, it makes good sense to shop resale. I always go to the Once Upon A Child stores to buy playclothes and the occasional dressy dress for the kids.
I have also found costumes, excersaucers and strollers cheaper here.
Be careful what you try to sell. THey are pretty strict about what they can take, and don't always offer a lot in exchange for what you bring in.
Take your things to the Julian Center and get a tax deduction worth more and help a good cause.