Used clothes don't make you money. They might give you penny's on the dollar.
I have a friend who only buys name brands for her kids. When they outgrow something she sells it on Ebay. She might spend 18 hours on one item with taking pictures, uploading the pictures, writing the ad, posting them, dealing with everyone who wants to see a picture from this angle or see the inseam seam or to see the insides of each pant leg at the hem to see how worn they are, or to argue with someone who wants to buy a pair of pants you have listed for $2 and they want them for $.50 and no shipping and handling.
Seriously, she's had gold and diamond bracelets on there with certifications along with the official insurance quotes and had them on sale for less than half. One bracelet was certified to be worth $850 and she had it for sale for $300. She only sold it because her house payment was overdue due to financial aid coming in late.
This one guy offered her $75 for it. It was emeralds and sapphires set in platinum and gold. Gorgeous. She told him no but if he really wanted it he could buy it for $300 because that was her bottom dollar. He continued to send her messages requesting she come down on the price.
He turned her in for not selling it to him at the lower cost and she got in trouble with Ebay. They set her limit of items back to a lower amount because of a complaint they considered valid....to buy something that had a guaranteed value of $850. He wanted it for 90% off. There was no way she was going to let this go for a penny less than $300. She took it off and he wrote nasty messages to her again and again wanting that bracelet for less. She had to close that account.
So I suggest that if you do sell their clothes that you simply find a local consignment store and take in the limit amount and see what they take. Because they might not take half of it. I've taken in things I just bought there a few months before and they outgrew it. Then when I brought it back in the same shape it was in when "I" bought it they say it's too out of date and it won't sell...so even then it's iffy.
Otherwise just save yourself a lot of time and effort and give them away to someone younger than your kids. Pass them around.