If I were a VP (Vice-President) who had to book passage for a world that was 3 parsecs away, providing materials necessary for production on this world, I'd realize the following would have to occur:
A) There would have to be deliveries between that world and this one. As such, it is probably a purpose built freighter that does not carry passengers of any kind. In a pinch, I'd consider taking a freighter if no other transport could be arranged.
B) As a VP, I'd also probably call personnel department and tell them I need arrangements made to travel to World D, and tell them to make the arrangements and call me back when they are made.
Now, lets shift to the poor person who now is tasked with making arrangements for me. That person would likely tap into a computer network, and either locate a service agency that handles making arrangements, or expedites the process by acting as an intermediary between the starport denizens, and the world's population. The intermediary would know which ships were in port now, which ships were scheduled for what destinations, and what the current status is of any given available stateroom. How would that interface agency know what staterooms were available, and so on?
The only answer I can arrive at, is that the interface agency (or agencies plural) would have to be told what the ship has. Since a stateroom can be either high or middle, it would seem to me, that the only real difference would be based on the following:
Ship captains upon hitting port, would announce what their ships are capable of carrying to any given destination. For instance, a ship with 6 available staterooms, 20 low berths, and 72 dtons of empty cargo area capacity, would do so within the first 24 hours of having landed in port. Chances are too, that there are actual "laws" about mis-representing whether or not a stateroom is to be classified as High or Middle passage, which would require that the captain state up front, whether or not he has a listed steward in his crew manifest. As this can change with but a blink of an eye, a ship without a steward listed would be automatically listed as unable to carry high passengers, and a ship without a listed steward as part of its crew manifest yet carries high passengers could be targeted for violation of the laws regarding misrepresenting a cabin.
So, as part of the admin duties of the captain, when he finalizes his debarkation proceedure, must have a listed Crew Manifest including any working passage crew, and must be able to prove he has a steward if he has a listed manifest carrying high passengers.
But that leaves us with the issues of how one knows when a cabin is available or unavailable. The best I can suggest, is that a central agency holds the data, and licensed travel agencies can broker sales of available stateroom and/or cargo space on a first come first serve basis. The ship's captain automatically constitutes a licensed individual able to broker space aboard the ship. Such a central authority would also require that once a "bid" is emplaced for potential sales, that the captain has to ratify the bid before it is listed within the system as being "sold" and subsequently, no longer available.
Now, the clerk from Widgets-R-Us probably has to check, or already knows, what worlds are required to be travelled to for the Vice President of Procurement to arrive at his destination. Due to the rules emplaced with BOOK 2 STARSHIPS however, no one knows at any given time, when a ship will arrive, and no ship may book passage to a destination further than one jump away. So the clerk knows that in order to secure passage for his boss, he has to no only secure passage on a ship outbound from World A, but he also has to make arrangements via some proxy organization, to secure tickets even if the boss is not physically present on the world at the time.
Example? Monday of Week 1, the boss buys a ticket for ship outbound to World B, but it won't leave until Wednesday of week 1. It won't arrive in-system of World B until Tuesday of Week 2 at the earliest, or Thursday of Week 2 at the latest. If a ship arrives at World B on Thursday of Week 1, but expects to leave Thursday of Week 2, a proxy agent could secure a ticket by buying on behalf of Widgets-R-Us on Monday of Week 2 in expectation that the passenger will arrive in-system in time to leave by Thursay.
So how might this proxy agent be made aware of this information? If mail is sent out on Monday night's outbound ship heading towards planet B - some 2 days ahead of when the Boss's (ie your) ship leaves.
Now, how might this proxy agent operate? NO REFUNDS! If you make arrangments to purchase a ticket, it is purchased whether you show up or not.
Now, do this for each and every world that you have to transit through. Toss in the fact that you may have a layover that is a week or longer as you wait patiently for a ship to arrive that will be heading out towards your final destination. The "one jump" at a time limitation for generating potential passengers and freight/cargo will run into problems however...