> I would still call those (peer to peer) exchanges

Stuff can be two things

I like wikipedia's definition of the dark net:

"A dark net or darknet is an overlay network within the Internet that can only be accessed with specific software, configurations, or authorization, and often uses a unique customized communication protocol."

Tor and i2p hidden services certainly qualify in my book. And if the hidden service lets people list items for sale, it's a martetplace on the darknet. A DNM.