For trade, until FL gets on with the space truckers book I plan to use the traveller trade system. I
Which one? CT core (Bk 1-3, ST, or TTB), CT Bk 7, MT/TNE (close to Bk 7), T4 (close to MT/TNE), T20 (close to Bk1), GT core (somewhere between vague and broken), GT:FT (totally unlike anything else Traveller, and almost unplayable except for the most devoted number crunchers), Hero Traveller (B7, MGT (not related to CT B2, B7, nor GTFT), or T5 (Distant child of Bk 5 via T4).
Could have saved yourself a whole load of typing by just asking 'Which edition would you recommend'
My personal go to is Mongoose Traveller, the one by Gareth Hanrahan, I think publication was 2008.
I run the COTI boards for Marc Miller. I provided some assistance to Gareth on that MGT1 Trade. (Which is what got me a special thanks in the credits). It's a worthy system, and a very reasonable approach.
I do discourage Traveller fans from just saying "Traveller" - because there are all so many editions with so many differences (and Marc is planning another one - "T8: Traveller for 8+" - a simplified game for 8+ year olds)... Besides, if you think MGT trade is complicated, you need to utterly avoid even thinking of GTFT....
I will note also: Traveller (except GT) assumes about Cr1000 per Ton as freight rate... my assumption of 80% full (comparable to type R Traveller freighters using T20) and a single calss III cargo module, puts the shipper paying between $100 and $500 per ton. So, you may want to account for that. Also, the default assumption is that the real profit is in speculation, but given the FTL comms in Alien (see p. 170, right column, paragraph on Communications), speculation should not be dominant.
As a historical phenomenon, Speculation as a percentage of operational space rapidly drops along with comm times. In-country speculative trade on the Eastern Seaboard (US/Can) dropped rapidly once the telegraphs went in; it was cheaper to order what you needed to arrive shortly before you needed it.