The American Chamber of Commerce does a charity donation drive once a year around January. When the time comes, I’ll post a notice here if you or anyone else has things to donate.
I believe they work with schools in village areas of western Sichuan. I’ll provide more details as I learn about them. Hold on to your stuff!
I know that in Shanghai they have thrift store type second hand clothing shops, (Cue Macklemore – Thrift Shop), but not in Chengdu that I know of.