One new twist. At first blush, maybe it violates KISS a little too much, but roll it around on your tongue a few moments to get the full flavor before passing judgement. I think it will seem simple and attractive to players. :->
As my post above, have a cargos table for export goods. Also have a very similar table to determine what import goods are in demand during a given week at a given starport. It is skewed to reflect what a UWP would want to buy, just as the export goods table is skewed to what UWPs want to sell.
From the player perspective, roll 2D to see what cargos you can pick up, then travel to the next system and on arrival roll 2D to see what they're willing to buy from the PCs that week. That meets the KISS standard, right?
PCs will only very rarely find an exact match between the cargo(s) they're carrying on speculation and what they can sell, but a modicum of sense and a couple of minutes looking at the trade tables should ensure that most of the time they can sell most of what they're carrying. If they have unsold surplus, they can choose to keep carrying the unsold goods to their next port, or see below for alternatives for getting rid of cargo when there is no immediate local demand.
The player(s) in the group who do the little bit of work of scanning through the tables to decide which cargos are good risks can make the decision by the seat of their pants and still make a difference to their profit margin. They do not need to do mathematical analyses that are difficult or tedious for many persons. OTOH, those who enjoy that kind of thing will find that, since they have to roll dice at both origin and destination to see what cargos are in supply and demand, they won't be able to guarantee a cash cow simply because they have the mathematical ability to milk the system. Sure, ability will help, but there will always be risk.
If each table is skewed appropriately for the types of goods that would be imported/exported for various UWPs, then flows of goods and capital should give sufficient
appearance of being balanced for most persons' purposes.
This should result in fewer players having problems with overstretched disbelief suspenders.
There is relief for pressured sellers! If the players are unable to find any purchasers for the types of cargos they are selling, they can wait until the next week and roll again. Or, they can sell to a local factor (NPCs with trader and/or broker skill) at a substantial discount on the nominal value of the cargo. Blanks yet to be filled in on how much the discount should be, and how to determine any modifiers to this price. There is a good opportunity for role playing the NPC encounter(s) with the factor(s), but it isn't really required so referees not so inclined can easily skip it.
Subsidized merchants and cargo carriers who are travelling regularly scheduled routes in service to a shipping line should get some advantages to finding buyers for their cargos, but also more limit to potential profits. This reflects prearranged cargo shipping, as well as standing relationships with factors and other buyers and sellers.
This game mechanic probably encourages PCs to try to create standing relationships with particular NPC buyers and sellers on any world they expect on a recurring basis. Which, it seems to me, Is A Good Thing. There should probably even be positive DMs for buying/selling with merchants where there's an existing personal relationship.
This personalizes and fleshes out the selling process in a way that tends to increase players' interest in what they're going to be feeling the need to do in any case. They may as well enjoy it, right?
The funny thing is reinventing this idea now in 2005. When first reading the LBB trade rules in 1977, it seemed like that was what they were about to say, and I was surprised and a bit disappointed when they didn't. I'd forgotten the idea completely until just now.
-Laning
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"Reality is that which, when you stop believing in it, doesn't go away." -Philip K. Dick