Back to the question at hand concerning Trader's Guilds, I spent a lot of time looking for additional references than the one displayed. After journeying down that rabbit hole I decided to make this a "IMTU" option.
What I do is offer membership to those characters who were in the Merchant Marines, or whatever may be similar. I set the yearly dues at 1000 credits. The benefits are:
(1) 10% off the cost of available equipment,
(2) reduced rates for ordinary lodging (set to availability),
(3) 10% off the cost of any equipment leased no more than 40 days.
(4) Priority work for starport mechanics on repairs and refits. (Essentially member's get their work done before non-members).
That's it. I hope this helps.
If it helps even more, the Trader's Guild I'm currently using in one Campaign, is an organization that will also let members know what items are deemed contraband at any given star port, lets captains know of any recent news from recently arrived members. It provides a service that shows black-listed crew members and why they were black listed as well as who blacklisted them (and when for that matter). It also lists the reliable transport services on any given world - giving a blacklisting for various transport (logistical) companys that have burned members in the past. The Fee I'm using in a GURPS TRAVELLER campaign is 5,000 credits for the first 6 months, and then will drop to a lesser yearly fee after the probationary period is over. Five "incidents" reported against your ship (late deliveries, legal judgements against your ship for damaged freight etc) will be cause for termination from the Trade League. Last but not least, it acts to help find available cargo on any TL 7+ starport world for cargo available outside the starport (requires a computer system and databases to function effectively) but is unable to provide that service on lower tech worlds.
And finally - it acts as an information clearing house of recent events regarding world classifications. For instance, in my campaign, members of the ruling class of Adibicci sanctioned a hit against an Imperial noble from Lunion while he was visiting their world and outside of the Starport boundaries proper. First a sniper attack from imported weaponry from off world, and then a "terrorist" attack via grav vehicles launching weapons against a starship that was known to be carrying imperial nobles aboard. The Governor of the Starport attempted to deal with the issue, but found strong evidence that the very people in the government were the actual culprits behind everything. When he couldn't gain cooperation in the investigation on Imperial noble's lives, he declared his starport and the world itself, to be a Red zone. This cut off the "off-world" trade bringing in higher technological products in, and starved the world of incoming foreign currency.
When yet another attack was made in the form of Alkaloid Poisons against a Baron of Lunion - and the assassin was captured and interrogated, the Duke of Lunion suggested to the Imperial Navy, that a warning raid was required against certain parties in the government of Adibicci. Some of the Imperial Marines used in that punative raid, were instructed to show up, and while the "customers" were asleep in their presumably SAFE mansions - leave evidence that if the Imperium wanted them dead, it would be only TOO easy. Heads of recently slain animals were left in one of the homes, while empty cartridges of a military rifle were left in the bedroom of yet another errant member of the Adibicci government. In the end, the Adibicci Secret Police (ASP) were told to stand down and cease their off world objective of trying to assassinate members of a certain noble family.
That Red zone classification lasted nearly 6 months until the offending world government began to see reason.
But that is why the Trader's Guild exists within my campaign world. It does offer SOME benefits, mostly information, and acts to weed out some of the bad players in the world of trade.
Addenda:
Oh, I almost forgot! The Trader's Guild only maintains offices on Worlds with Class A or B starports and works within the same building as the Traveller's Aid Society. That is one reason why they charge so much for the services. It costs money to have a physical presence within the Third Imperium.
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