thanks for checking it out! You are correct, at the moment you can only sell stuff in systems that also sell it, but I'd say the trading system overall is still a bit undercooked. The prices and distribution of goods is essentially random, and eventually it should be a bit more logical and you should be able to sell anything anywhere.
You can use the inventory tab of the in-game menu to mark the best known price (you only know prices on planets/moons/stations you've previously landed on) for cargo you're carrying on your map, to make trading less about guesswork.
I do also have some plans to reduce the likelihood of getting insta-wrecked on jumping into a system, mostly by making sure the player spawns somewhere that's relatively open and not right on top of a firefight. I may even mark systems as warzones even before you've discovered them, to at least warn you to maybe route around them