Plenty of people just want to get rid of stuff. They need the room or they need to move and they don't care what happens to it. If this no-one takes it it'll go to landfill.
He's providing a service to them. He takes the stuff away, and doesn't charge them to do so. He might even give them some money. (Compare that to hiring a dumpster / skip.)
I don't know what Americans throw in landfill but in England there's plenty of furniture or working electrical items that end up in landfill (or the modern equivalent).
But not if they were on craigslist in the beginning and apparently there are buyers. From what it seems to me, he just tries to be the fastest buyer rather than taking things that can't find buyers
He's providing a service to them. He takes the stuff away, and doesn't charge them to do so. He might even give them some money. (Compare that to hiring a dumpster / skip.)