Re: desperately poor people
There's absolutely people in the middle here. Smelters might buy a tonne minimum lot. No hand miner is going to collect a tonne: that's $16,000 at current rates. So there will be a chain. Small dealers you can sell a kg at a time to. Others who buy up that in 50kg or 100 kg lots and so on. And every level will get its slice. (Please note, I don't know the actual numbers, just the system)
But not quite like fences, in that the middlemen get near all of the value. There's enough competition in those chains that margins aren't all that wide. there's also a number of smelters, both legal and illegal. So no one has a squeeze on the prices.
One estimate I've seen (from Friends of the Earth actually) is that a miner can make £12 a day. Hard work for shit money, undoubtedly, but that's about $18 more than the water buffalo option provides. For that they might be collecting 4, 5 kg of cassiterite a day.