I really think the only way to make sense of Traveller trade is to posit that there is a high-volume, regularly-scheduled trade network between the larger and richer worlds. This would be analogous to container shipping on present-day Terra. The ships involved are big and specialized, and so are the port and transshipment facilities they use. Ordinary players don't interact with this system at all, beyond obeying traffic control instructions to avoid colliding with any of it.
Meanwhile, at the smaller scale, there are the ships characters care about -- tramp freighters, subsidized merchants serving smaller and poorer worlds, and so forth.They work the niches that the big container-shipping network doesn't occupy, on the margins (of population, economics, the law, or whatever).
I live in Los Angeles; the port of Long Beach is one of the wonders of the world, with dozens of giant container freighters in port at any given time, and huge cranes, trains, and fleets of trucks moving freight around on an unimaginable scale. There is also a modified WW2 LST that comes in from the tiny village of Two Harbors on Santa Catalina Island once a week to drop off a load of trash and pick up a load of food and other supplies. The PCs are on the latter. :)