Shops are hardcoded and the only way to get a hold of the code is to break into Derek's special hardcode safe in his office. Hf, Frog and I are planning a caper, but we still need a wheelman if you are interested.  

I am going in with 2 kilos of C4 on me chest and a dead man's switch. He ain't taking me alive. No sir. 

Glad to hear that frog is for a change here. I'm game to be the wheelman to get this change moving

How about no selling to people unless they sign an Alliance? Even if I sign a Non-Agg Pact, I don't want to be selling Lord Verga a Greatsword...

Except isn't making an alliance really far into the tech tree?  Also, I think the stuff in post #8 could work.  Some items are so expensive, if they had an extra 20% or more added to the purpose price based on predisposition, it's basically out of reach.  And I would also add the ability to set our own predisposition, so we could make ourselves appear hostile to certain nations (probably at a penalty to their relations with us) so they couldn't buy our stuff.

What do you think of this, Seanw3?

PS: The Ideology could provide a multiplier to the rate as set by predisposition as parrotmath suggests.

Actually, an increase based on relations would be fitting. The AI actually has a two-way relation mechanic with the human player. We might try a percent increase in costs for every diplomatic point you have. So having a net of +4 relations would be +20% shop prices. +8 net would cap the bonus at equal to my own costs, so you can't game the buying system on good relations. Then for 0 net and lower you start to pay massive sums. If Lord Verga paid 800 Gildar for that Greatsword, I would probably gift wrap it myself. 

This could also be a good system for trade. The better relations are, the more trade bonus you get. And Tarth gets a natural +10% for them and -10% for you because they have so many bandits in their faction. 

Now, if they just give us some access to shops for us to do this for mods. There is so much that can be accomplished here that I would like to see it happen.

In fact, if you choose the bandit sovereign background, this could be one of the benifits added to it a +10 % or - 10 % to trades.

I think in order to buy anything from another faction, you would AT LEAST need to be allies.

I too would love for more of the Shop functionality to be exposed to modding.  At the very least, I'd like to be able to alter the number of items the shop will hold and have direct control on what is sold in the shop as well as access to some way to control or alter the prices of items.  I have created a lot of replacement items for my faction but I can't prevent the items I've replaced from showing up in the shop and with the max number of items for sale hard-coded that means some of my higher level items might not be purchasable late game.