Maybe part (or all) of the cost of anything you buy in a foreign shop should go to the foreign sovereign's treasury? Then you're gaining a benefit, but it's also benefiting your potential opponent(s).
Also, if it isn't already the case, limit things we can buy based on relations status - e.g., if relations are worse than 'cool', no shopping in my territory, if relations are between 'cool' and 'neutral' you can buy tier 0 items in my territory (clubs, staffs, pork, etc), between 'neutral' and 'warm' you can get tier 1 (things from the first two or so levels of techs in each tech tree), so on and so forth, modified by what has been researched so far (i.e., if I just researched 'Charms' and I'm hovering between 'Warm' and 'Neutral' with Altar, maybe Altar doesn't get to buy my new toys), and possibly with a bonus for having a 'Diplomat' sovereign. So, if all I have are items from the 'tier 1' zone, maybe they won't show up at all for anyone not an ally of mine, but if I've researched the entire tech tree, tier 1 stuff will be available to almost anyone who wanders into my land.
One thing, though: if shopping in other factions' territory starts feeding them part or all of the cost of whatever is bought, item costs might need to be changed - the costs on some of the items are rather ridiculous and can represent a huge investment of time to acquire (even late in the game, I don't think I've ever had more than about 50 gildar income per turn, yet some of the items cost 1000 or more each). For that matter, I wouldn't mind seeing the costs change regardless (nearly 1000 gildar for a Boreal Blade? At that price, I'll just use that mace I looted from the Troll Camp, and I'm sure to find better weapons in loot from tough monsters and higher-level quests). Having the items cost five times the listed value might be okay in some cases, but it certainly discourages me from ever buying anything beyond basic equipment for any of my champions - chain or leather plus whatever they loot is going to have to do for them, unless I have money to blow later on for something better and for whatever reason haven't found anything better from completing quests or looting monster lairs.
