What to do with leftover stock?

Hi! My dad’s been a seller on Amazon for YEARS and has a really big collection (our entire garage) full of boxes with toys/games/cds/books/tools etc that he has been selling over the last 10+ years. He’s retired now but still needing to make money, so he wants to make whatever he can from what he has and clear up his space so he can eventually move. I was wondering if there’s anywhere he could just sell all his stuff bulk and get it over with? Any advice welcomed and appreciated!!