While we sort out the flow i am interested in the ships. Me and my brother talk about this from time to time. Are these major cargo vessel that basically hover over a planet and suck up all their goods and people every year? or are they high jump Jump Tender ships that jump in burn to the highport and take on different cargo ships or other ships needing a lift. I like the tender ships because it allows for less jump capable ships to move between routes or move faster if they have the credits. A j-4 Jump Tender could care 100,000 tons of ships or 500 free traders. or 10 massive liners/cargo shipd
I think the actual ships would be up to the gamemaster, universe-developer, demigod, whatever we want to call them. A tender has clear advantages, but there are only 7-8 blue "Major" routes in the entire Marches, one serving Darrian, one serving Rhylanor, one serving Trin, four in the Mora subsector and one leading out of Mora subsector to the neighboring sector. Some, like the Rhylanor-Porozlo route, are huge-pop worlds, and much of the trade is likely to stay within the two worlds of those routes because the worlds involved have huge pops and likely have equally huge appetites for the trade that's coming in, so big ships could serve those. Some, like Trin-Ramiva and Fornice-Maitz, look more like trade is passing through toward someplace else: tenders would work well for those routes, with the riders hopping from one tender to another and possibly jump capable so they can hop the final route of a feeder or minor without needing to spend time moving loads. It'd have to be a pretty nice cargo to justify the expense of multiple jumps, but I could see some pretty nice cargoes coming out of a TL15 major world.
If you have 10,000t-100,000t tradeships transporting TL15 manufactured goods around the Spinward Marches in the volumes you suggest I have a hard time accepting that TL15 gear is anything but common on every world in the Marches.
If these tradeships transport TL15 spare parts to every type A and B starport in the Marches then every type A and B starport can maintain ships up to TL15 and therefore local world TL doesn't count towards starport TL - which provides evidence for my hypothesis that all starports should be the TL of the ruling polity within the 3I setting.
Well, lessee. I'm running off the wiki data, and I'm going to assume that most or all TL15 exports stay within the Imperium, first because the four major worlds that produce them are deep in Marches Imperial space and second because some of it would be restricted for export to prevent items and knowledge with potential military applications from ending up in the wrong hands. The four TL 15 Imperial worlds account for 44% of the Imperial Marches GWP and 49% of the Imperial Marches interstellar trade economy. If we drop my assumption and take the Marches as a whole, it drops to 31% and 37%. Interstellar trade only accounts for 0.4% of the economies of the four worlds in question, so serving that trade is not terribly difficult for them to do. Of course, some of that stuff is not high tech gear - it's going to be common stuff produced using high tech processes to increase output and decrease overhead, in order to achieve a higher profit margin.
So, yeah, TL15 gear ought to be pretty easy to come by, although it's going to be pricier than it would be if you were on Mora or Trin or suchlike. Of course, some world governments may restrict imports to their own citizens for their own reasons. Consumer goods like entertainment devices are one thing. Having your electrical grid dependent on parts that have to be shipped in from 20 parsecs away, or giving your citizens access to technology that might be used to undermine local government (like devices that could be used to counterfeit money or IDs, or electronics that could be used to tap or hack the local information network, or 3-D printers that could be used to produce contraband weapons) might be a very different thing in the minds of the local government leaders.
Add: Oh, I should add that interstellar trade only accounts for a tiny percentage of most world economies, so while the goods are getting there, the vast majority of the stuff being traded back and forth within a given world is local product. I figure you've got something like a starport import store where you can go to for the really cool stuff, but away from the starport it might be a lot harder for people to get hold of the cool gear - and it's more expensive to them in terms of percentage of their per capita income than it would be for some feller on a TL15 world, so they might settle for cheaper low tech. A TL7 boom box might be easier to afford for a local than a TL15 wristband-controlled polysymphonic earbud.
