It would be hard to allow buying but not selling. Cargo amounts could be checked and re-added then credits edited as well, but it would likely be a "jarring" manner that's messy and not totally obvious to the player.
Better to just block commodities if you don't really want it.
There is a way to block out the commodities market. Save Anywhere OXP uses such a method to prevent buying and selling at the main station when you're temporarily placed there to save your game.
Unless I set my ship up to act as a Taxi, what reason would I ever have to visit a Taxi station?
I suggested small quantities of commodities (gold/plat/gems) or at least a place to sell some commodities (food, liquors/wines) only as a means of giving me something to do there.
Are there or might there be missions associated with Taxi Galactica?
Agreed I didn't ever want commodities to be bought/sold there, as I dislike there being places in the same system where you could trade back and forth and make 'easy money'. It discourages players, especially 'Jamesons', from going out and exploring the ooniverse. There is only one 'special mission' available in the OXP after completing so many (think its 5 but I'm at work and cannot check) contracts. but I have always planned to add more. Rather than devoting time to tweaking the commodities market I think I'll continue implementing more missions with more variety than just ferrying people around.
Just a mention that my (surprisingly popular) YellOo Cabs
make excellent Taxis, and you are welcome to incorporate them into Taxi Galactica if they would be of use.
If you wouldn't mind? Would help to create a variety of Taxis, rather than just the Taxi Adder. I have been using dertian's Taxi Adder (which is a modified Griff Adder) as it fits in with my 'Griff-ed' ooniverse, but as I have noticed from my Flyable Thargoid Warship OXP, where I've offered two editions of one standard and one 'Griff-ed', the standard OXP has only been downloaded a few more times than the Griff version, so perhaps I could offer Taxi Galactica on two different editions and bring back the old models in a Standard Edition and use the dertian retextures in the Griff Edition, as I know dertian did a retexture for the YellOo Cab as well.
One more idea.
Real taxis have comms so that they can be notified of customers when they are driving. Taxi Station could offer a buyable equipment that would alert player when there is a customer waiting. These contracts could be randomly generated in-system transports from one station to another or maybe to the planet or moon surface depending on the oxps installed. When flying in a system you might get a comms message saying something like "Customer waiting for a transport from Coriolis Station to RSS Station, do you accept?" If you want the job, you would have to accept it in 1 min. Accepting could be done with primable equipment and the payment could be fixed to something like 100 cr.
Long ago I wrote the Planetfall Taxi add-on OXP which does most of that already (between station and planets/moons). It's somewhat in need of update and overhaul, but if people want to use it (subject to its license) as a basis of such an expansion to one or more of these taxi OXPs then feel free to.
I was going to say to spara that I'm not sure how to go about having passenger contracts to go to somewhere in the same system, but I too had forgotten about the Planetfall Taxi missions. I'm not sure how you did this Thargoid, as Taxi Galactica uses the same method of passenger contracts as the standard passenger contracts (using the player.ship.addPassenger() function). I will have to take a look at how it was done in Thargoid's OXP and see if this is incorparable into Taxi Galactica.