I like ThredUp too, although it seems a bit more picked over when I shop for little boy clothes. It's the same situation at the thrift stores, too. I think in general people buy their little girls more clothes because of cultural cues, so there are more to donate and they are less worn by the time the girls outgrow them.
I have made decent return selling good condition hand-me-downs on ThredUp. One time there was a delay in processing my bag and they automatically gave me a $40 credit because of the delay. I know the clothes I sent would not have made that much.
I have not tried the grown-up side of ThredUp - I'll have to check it out.
Does anyone know a good source to find used little boy pajamas? I don't see them on ThredUp. At the Goodwills in my city they separate the pj tops and bottoms when sorting and I have never been to find a matching set. Also they tend to be very worn by the time they get to Goodwill. I don't have time to go to garage sales. I have been buying pj's at Costco, but am hoping for a cheaper source.