M.J.
I've only done it once. I like to be a seller at the school resales instead.
You need to make an appointment to take your stuff in there. They go through it and will give you a computer printout of the prices they will offer and you can take them or not.
I found the amount they offer is not great, but I only had baby stuff and boys clothing. I don't know how it works with the girl stuff. A few things I just wouldn't hand over - Diaper Genie and Exersaucer they only wanted to give me $8. That would be fine if they were scummy or had some wear, but I only used the stuff on 1 kid!
The school resales you can name your price. I've found that since the school is getting part of the cut, people are willing to pay a little more. Like you can price blue jeans at $5 or polo shirts at $3. That's not what Once Upon a Child was offering.