I don't trade or use bitcoin but I understand the mechanics of...

I don't trade or use bitcoin but I understand the mechanics of the encryption system. It's based on public key cryptography, which underpins the entire Internet these days.

I can't comment on whether he's telling the truth or not but AFAIK it is feasible to do what he says. To do it, you need to find the private key associated with the bitcoin wallet, which if somebody was foolish enough to save on their hard drive or a flash drive (in a text file), you could definitely recover. You don't even need the original wallet software, you can just download the same wallet program and use the private key to activate the bitcoins. Assuming the person who threw it away didn't have another copy of the key, nobody would have been able to use them in the meantime, so those bitcoins will be sitting there waiting to be activated.

In the early days of bitcoin when it was worth a fraction of a cent, it's likely people did store their private key on their hard drive, because you wouldn't be too worried about someone finding your private key and stealing your near-worthless bitcoins.

The bitcoin program does not store identifying details of previous owners - it's designed to be anonymous. But it would still technically be illegal to do what he's doing because he is accessing someone else's private data. If I were him I would be keeping a lid on what I found, lest an old client find out. They'd have good grounds to go to police about it and demand their data back, including their bitcoin key.

So it's certainly possible. Whether you believe it or not... I have to wonder why he'd go to the US to sell his bitcoins when he could do it just as easily from Australia. I also wonder how many old hard drives would contain a couple of hundred bitcoin - that would be from the early days of bitcoin when mining them was quick and easy, and almost nobody actually used bitcoin in those days except hardcore nerds - people who would have been pretty cluey about the need for security. The chance of finding even one, let alone more than one, old bitcoin wallets with unsecured keys seems pretty low - even in a shipping container full of old drives.