So version the magnet link (similar to git) and always serve the latest magnet link via a DNS TXT record.

where does the version control repo live?

Excellent point. I'll continue to think on this!

You might be looking for this: https://wiki.freenetproject.org/Updateable_Subspace_Key

It might be that a blockchain is the way to go.

Bitcoin lets you store 80 bytes with each transaction. Maybe that isn't enough for a magnet link and description, but you could use multiple transactions. Maybe there is more optimal blockchain design though.

Ideally you'd want a way of validating the content as well.

Or just host the site in the Tor network.

A hidden service is still centralised though, right? Given the current chasing of The Pirate Bay, I'd expect this hidden service to get the same scrutiny as The Silk Road.

My take on The Silk Road was that whilst it was quite possibly bad operational practice that took it down, it could have yielded to traffic analysis. I'd expect the same fate of Tor Pirate Bay.

But opsec can be improved upon, with enough care in how you host your torsite, what software it's on and who manages it, it should be possible to run it pretty safely, save for an attack against Tor.

I imagine the third generation black markets on Tor will last longer than the second one did