D.P.
I looked into a resale shop once and brought it all home & set up an eBay account. Kids clothes go like lightning on eBay! BUT--the money will come in over time whereas if you do the event, you'll get it all out of the house & get the cash in that day/weekend. A matter of personal choice I think. I like eBay, especially for my son's little worn, pricey shoes, coats, etc.
Oh--not sure about the other brands you mentioned but Gap isn't that "high end" on eBay--so maybe resale is better for all that? Not sure. Good luck!