1st of all love the idea of this mod but ... i personally don't like the idea of you actually having to go in the the area to look what you need and then going back to base to make / pick up these items and haul your ass back to “pay”
So here's a suggestion if you like it AWESOME ! if not then there's not much i can do about it :) your mod your rules.

And a note about all of this: its way longer than i anticipated hope youll like at least some of it and i hope that some itsy tiny bit might get actually realized :) all tho i have my doubts because if all of this would be actually possible to do then it would be also hell to balance and i'm not even talking about other mods integration ... so it's just a thought ... maybe some inspiration but not much else :)

Anyway here it is:
The buy process would be made via currency not items them self
this currency would be generated by “inserting” items in to building where each item value would be determined by its raw items value and little by little multiplied by its total craft time (let's say 5% of that craft time would be added as bonus value) and by static bonus for actually crafting step (3 step crafting = 3 static bonuses (idk 0,3 per craft)) so in the end the calculation of value would be something like this

Item value = (basic resource value + ( (craft time * 0,05) + (static per craft bonus * number craft operations) ) / amount of items crafted
Iron plate = (1 + ( ( 3,2 * 0,05 ) + 0,3 * 1)) / 1
Iron plate = (1 + (0,16 + 0,3)) / 1
Iron plate = (1 + 0,43) / 1
Iron plate = (1,43) / 1
Iron plate = 1,43

Note: After looking at the result i think that just total raw time bonus would be sufficient as bonus value cause it will scale quicky up as tech advances.

Currency itself would be stored as gamestat shown in GUI as “<currency name>: <money amount> ”. or alternatively visible by entering some command (/money) or worst case scenario by handling items generated inside the building where you would put the produced items for “sale” (1 full number in building = 1x currency stackable to let's say to 999 999 (hope that would fit the icon square)

Territory value would be determined primarily by resources present on that specific territory you would have to pay lets say something around 20-40% of resource value in that territory (resources density increases with distance so would be the value of territory) + static price per tile lets say 10 “money” per tile
Also all of this would be awesome if it would be configurable in settings be it percent of “pay in advance” for resource present in territory, price per tile but also value of “basic items” default: if item then value 1 if liquid then value 0,01

territory buy: ideally this would be done by interacting with some entity in game it self at least building or ideally some flying drone where you would click on it see the value it wants and have option to buy. this drone would be also what attacks you (be it the drone it self or by targeting you for orbital strike) and it targeting you with red pointer to tell player pay or get lost and with it having little countdown on top itself while pointing at you once you enter. drones would be placed in each individual chunks and then linked if you have bigger territory than 1x1 where you would pay off 1 drone but (let's say you have 3x3 chunk as territory) 9 of them would get either destroyed or optimally fly away to space
Ideally flying entity to avoid bitters and set to invincible / unattackable for the same reason
Also absolutely perfect would be if you could access this entity from map view without you physically going there assuming the currency would not be items and maybe even then... the items maybe would not be problem if you can actually open the UI of that entity from distance

That's a great view of the mod! Thanks for sharing =)
When I develop this mod I opted to keep it simple, so il would alse be simple to mantain. Adding new entities was never thought in my view. But your ideas can also be implemented in a simple way: the mod already has all item pricing routine integrated and already uses "coin" idea. I even have another mod integration that also asks for money to claim a territory. Asking for an item instead of coins was an option to make it much harder and challenging. I can charge for money only, and add a custom keybord key to "sell" items and receive coins.
If the price is only coin, I can calculate terrain price on chunk generation and show the cost on minimap. But for items, I need to calculate and choose itens after the player goes there, so I will check at that moment what the player already knows to craft.

I got no problem with hard but i personally find it tedious not hard to deal with.
its tedious to go to the area to buy it mostly early game.

Example from my game i'm playing with krastorio which adds rare metals as basic resource and this recipe is unlocked from start because it IS just a basic resource and there's no point locking smelting recipes like that in research. Aaand i got rare metals in territory on top of me right in territory where i wanted my def. perimeter… i could have used “free territory” to close this huge gap in my def. but i did not want to (stubborn i know xD ) so i looked around and there was rare metal 3 territory to top and then 2 territory to right so i went up marked 3 territories for what i need … aaand .. die because i was too long in those territories witch added to my frustration because i now needed to go and grab my items … walk there … without exo, without car … just ever so slowly ... that was tedious not hard … there was no obstacles that could stop me it just took time and nothing else … also the tedious part is that even tho i die even tho i wasted time going up for my items … it was still faster than going 3x times to buy each territory xD

Anyway the point is that all of this could have been avoided if there was a way to buy the territory from map rather by going there witch i think is not possible to do with items and even if it were then i think money is still the better way because you can make it more expensive when just 3x3 territory cost 100k it's not a problem because you have all types of resources at your disposal … you don't need to worry about player getting stuck on specific resource as long as you have .. something .. you can expand and claim more that stone you have no use for ?? sell it … get more territory … you got stuck because you got not iron in your territory and you don't have money to buy neighboring territory which contains iron ?? you have coal ! sell it … you have trees chop them down and sell them … you still have options to go with which you don't have with items with money it can be hard … but with items you're just thrown in front of RN jesus and all you can do is pray that he will not screw you

Oh and also you mentioned selling key to sell items from player inventory i think it would be better to “sell” items in building (idk exchange storage?) because:
a) it's automated… factorio is about automation
b) prices will be higher and higher the further you go and let's say you're 100 territories to one side and you have territories which contains resources with value of 100kk+ and you would have to sell multiple player inventories worth of stuff to buy it … that's a lot of clicking

Hi. Give a look on the new version (a new mod option)

Taking a look at it now looks good only thing i would say is take a look at seller chest : when you pick it up you get iron chest not seller chest
and idk if its intentional but its not possible to sell wood and fish all tho its probably for the best that way :D

And i had to start a new game but that was to be axpected :)