https://lego.com/replay :

> Q: Do I need to clean my bricks before I donate?

> A: Thank you for the offer but no, you don’t. Give Back Box will be sorting and cleaning the bricks before donation. To help them in their job, you can try to remove obviously damaged or disfigured bricks from your donation.

People do try to throw them away. A good chunk of my kids' lego collection is a mixed selection of blocks we found in a skip.

I got 75% of a 7745 train, a ton of rails, a traffic light and assorted mystery trainish blocks from a bag that someone rescued from a dumpster. Unfortunately, they could carry only 1 of the 2 bags in the dumpster. I got it very cheaply, as they didn't think they would get much money from it. A lot of the rails were snapped. Some have white paint.

I restored it, but my heart wept for the previous owner.