I don't want to get overly involved in "economic" discussions since economics barely makes sense in the real world let alone in a fictional one... And I took years of micro and macro-economic academic courses... Thought that astrology was nearly as useful when you get past the fundamentals and basics, which people still argue about it (... with astrology, meteorology, or economics... LOL ).
The Traveller Wiki has this to say: [
http://wiki.travellerrpg.com/Starport]
The canon specifies differences between (A: SHIPYARD-manufacturing-production-fabrication) and (B: REPAIR-replacement-overhaul-upgrade)....
Either A. SHIPYARD or B. REPAIR tracks can produce new vehicles... The distinction to me is how much can be done from scratch and raw materials... In an age of 3D Printers, microfacs, autofacs, etc., I would think that on a non-profit level, just about anything could be fabricated or produced with enough work and the proper supply of raw materials (...and a disregard for profit). Here we go Metal Shop 101...
In real life, I picture it as the difference between a factory-produced mass-market vehicle and a "kit car." Both work perfectly functionally as vehicles, but one makes mass-market economic sense and the other is more suited to niche markets for people who pay more.
So, I would suppose that the relationship between starports and shipyards would be dependent on what kind of understandings of interstellar trade exist within the OTU. I think that Interstellar trade, without an ansible or FTL communications, would be limited in the banking (financial) and trade (shipping) senses... That's why Traveller, in my humble opinion, has such a lore of tramps, merchants and low-level trading... Only Megacorporations and really large entities can really make large-scale interstellar trading work on a decent profit level... And such trading would still only work with large volumes or high profit portable items...
But, of course, we're talking fiction... Just throwing out my ten cents...
Positive vibes to all!
Shalom,
Maksim-Smelchak.