I tried Once Upon a Child on 86th Street this summer and was shocked at how little they actually paid for clothing. Then, they turn around and sell them for a tidy profit. They bought an outfit from me for a dollar, but I found it on the racks on a later trip marked up at $7! Ouch.
If you call them, they will tell you the percent they pay. I want to say it was 20%, but I cannot honestly remember.
Also, they will only purchase itmes seasonally, so you might want to ask whether they are purchasing summer or winter items before lugging a whole basketful of clothes over there.
Other things to note: They do not take clothing with any kinds of imperfections, such as stains or missing buttons. You do not have to take the clothes hanging. They will do quick estimates in-store (which take an hour or two), or you can leave your clothes there for them to appraise in 24-48 hours. They can refuse any or all of the clothing you bring.
I hope this helps, but I personally would look into other venues to sell baby clothing.