All IMTU:
Player character merchants? They supplement trade and take on new cargo by adventuring, taking on risky ventures, and the occasional resorting to petty and not so petty crimes. Pirates (and PCs) are often ethically challenged merchants. All of which builds the campaign.
Pirate ships are there when needed by plot, same as for Navy ships, be they system or intrasystem, independent or funded by Megacorp or various governments.
NPC merchants? I'm not terribly interested in them except again as they hew to usefulness in campaign. The 'verse operates as it needs to and if I want to detail an Imperium wide trading scheme I'll use GT's Far Trader + spreadsheets, or go play some 4X computer game.
Also there's IIRC a two tier system of merchant trade. The huge ships, basically big containers with jump drives on the end, that ply the heavy routes, with megacorp backing, funding and protection. The pc level is far below that, smaller ships usually not devoted solely to trade, that take on what jobs they can to get by wherever there's credits to be made.
Very modern small wars / modern pirate & black or grey market trading, Hornblower and Age of Sail, Han Solo in Corporate Sector, Alfred Bester, Firefly, Cowboy Bebop et al. And yes I do like some background sense of a universe going on external to while still responding to the PCs actions. I just don't detail it too much or too far in advance, as the players' action will invariably go beyond what I have in 5 minutes of play or less. :shock:

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