Consignment Conundrum

Let's face it.  My first child was a clothes horse.  We got the best things from showers and at birthdays and with all of the awesome hand-me-downs, we were set.  Now I have a little mini guy who doesn't quite fit in the size or season I have.  While my husband's step-sister was in town, she told me about the large consignment sale in her area, where you tag things online, drop off, and get a great commission from your items.  As I thought about the boxes of clothing, stacked to the ceiling of our little basement closet, this sounded like a great idea.  We have a great sale close by.   Here's my issue.  What do I save and what do I consign?  We have more than enough for 3 children in each size, but I'm not sure what to hang on to and what to sell.  I want your vast wisdom.  Have you done a large consignment sale and how do you do it wisely.  Please comment and share!  I need HELP!