A system with a Starport that buys Spice and/or Equipment, and also sells
Uranium and Metal/Anarobes/Medicine (the more of these last three the better).
Mountainous planet within this same system
You place your Dome on the Mountainous (populating it be an option, not a
requirement). You rush the building of the Refinery. You then stock it up for
what it needs for the Atom Smasher, then rush to build that. Voila! You now have
a completely in-system trading setup. For those not familiar with how Atom
Smashers work, they be used for converting one resource into another, requiring
Uranium for it to work. Two Metal and one Uranium can give you two Spice. So,
you run Uranium and your cheapest resource from the Port to the Mountain,
convert them to the highest. Buy item at the Port, then go sell that and pick up
the next load to be converted. You will have to make an empty run to the Port
once every 3rd run to replace your Uranium. If you want to increase the profit
margin somewhat, convert Metal into Uranium to eliminate the need to buy any
Uranium (this winds up using a bit more fuel due to the number of trips to get
Metal being higher, but at 1 fuel per trip, it's not especially important).
Eventually, you will run the Port out of resources, but that takes quite a long
while you will have made a good fortune before that happens.