So, I was really quite inactive for months and now that I've returned it seems I have to relearn a few things ;) 

Anyway, on the Battlefield itsometimes says that dwellers of Io are looking for 100 Marble (or similar), but I don't see anywhere to actually answer this trade request.. Where can you give them what they ask for?

Apologies if this has been answered somewhere, but since sadly there still is no search function, I'm stuck looking through pages of Help questions and I didn't see anything there.

You need to move your squad to the central zone (portal) to trade with other moons. Trade requests appear once per day at the server midnight, one trade request per battlefield instance (we have 2 instances right now). In exchange for your materials, the dwellers of other moon will give you one of the items that are required to build high-level buildings.

Here's the news post where this feature was introduced: http://beastkeeper.com/thread/3290

P.S. Glad to see you back :)

Ahh thanks so much for the fast reply, Angel (and also the nice welcome-back)! :D 

I must say, once I started looking at my pets and the new features, I realized how much I've missed the site! I'm glad to see it went out of Beta and added so many new things! =)