I find it unusual that the best place to shop in the UIT is the capitol, especially since by the backstory, the UIT has a relatively weak central government. There might be cheaper prices on stuff elsewhere, but there's not enough of a price delta to justify visiting a whole bunch of stations to try and find a cheaper price.

So I would say that 1) prices for buying things in the UIT capitol should be more expensive, and 2) other stations should advertise some of their low-price items

Not sure what the best way to do ads is without making them annoying (aren't ads annoying by definition? :). The easiest implementation I can think of is to create an "ad channel" for each system. Stations should periodically advertise their low-price items on this channel.

This way players can join only the channels for their "home" systems. And, now that I think of it, this could also help traders out, if stations advertise cargo goods as well (possibly on a separate channel). Whenever you warp into a system, the mission tab could announce "Armaments advertised on channel 212, trade goods on channel 213".

I like the add channel idea. It would be nice to know what is sold, where it is sold, etc.

Of course there is nothing to say that the adds have to say where the cheapest goods in a system are.

There's going to be a news system implemented in two weeks. The Devs plan it to include trade price information.

I do agree with your concept of the weak central government. Dau L10 should have everything, but it should be more expensive than if you went to the production sectors.