It looks like they've set it up so the player who wants the resource must initiate the trade for it. I'm fine with that, problem is the AI hasn't asked for a single one of mine after two peaceful trade games where I met every AI and bought every resource. [Edit: They did and I didn't notice. It shows up in the notifications, diplo screen isn't opened when someone else buys a resource from you.] Like with diplomacy, the core trading system is there, the parts that seem to be working as intended are promising, they just need to clean some things up. 

They don't ask. As far as I can tell, here's how it works: any resource in your territory with the appropriate improvement is automatically up for sale. First come, first serve. When a resource is bought, its owner keeps the original copy and a second copy is generated for the buyer. This is normally the final copy from that node. The merchant culture special ability lets them resell resources they've bought from someone else, which creates a third and final copy for the new buyer. That's why when you're going through an AI's resources, sometimes it'll say you already own the trade rights to a resource you didn't buy from that player. They sold the resource to a merchant civ and you bought the third and final copy from that merchant civ.

When someone buys a resource from you, regardless of the type of culture you are, you don't get a big middle-of-screen notification like with other diplomacy, just a notification at the bottom of the screen next to to the ones for Era Stars. 

This also helps explain why resources seem scarce on the map. They are, and the idea is that merchant cultures can generate extra copies of other people's resources assuming the original owner and a third player are willing to trade with them.

If that's how it works, I think it has a ton of potential. Adds an extra layer to diplomacy and can give trade cultures leverage.