You would not believe what people give to Goodwill, and yes, they use it as a dump.
My daughter works for Goodwill running the local “shopgoodwill.com” online auction, Goodwill’s answer to ebay. She has sold mink coats, diamond rings, and musical instruments worth thousands of dollars.
Saw a complete silverware set, service for 6. Marked at $20. Upon closer inspection, I saw the 925 stamp on every piece. It was real silver. I brought it to the attention of a checker, who then called her sister to come in and buy it. She did not know I heard her. I went to another checker and purchased it. I waved to the one who called her sister as I was walking out the door.