Prior to the whole mess of the Jihad, ComStar kept Terra as a no-go zone. It was a big deal when Hanse and Melissa were given permission to have their wedding there. Getting into Terra wasn't too terribly hard, but once there you basically never left. And if you did manage to leave then ROM had a grand adventure hunting you down and getting rid of you.
Per the ComStar SB p. 86-87 and Fall of Terra p. 56 to 58, the Terran System was actually a major trade hub. Yes, accessing the planet Terra was "by invitation only," but it wasn't that hard - Max Liao (secretly) met with the Wolf Dragoons on Terra to secure their contract.
And while Terra proper might've been hard to access, "
Terra generates a surplus of goods that are sold to trading consortiums across the Inner Sphere. Despite being of no better quality than goods produced elsewhere, items labeled 'Made on Terra' sell for a premium....The manufacture of military weapons remains a major Terran industry. Only since the battle of Tukayyid has Terra's exports dropped, and that only briefly because of the need to replace ComGuard losses. ... [after WoB's takeover] the cities of Berlin, Chicago, Calcutta, and Wuhan are major black-market centers."
Aldis Industries, famous for numerous combat vehicles like the Behemoth, Demolisher, and Schrek tanks, operated from Terra throughout the Succession Wars.
Terra didn't have an economy based on interstellar trade. The Sol System was pretty much completely self-sufficient.
The Terran System was self-sufficient, but it was a noted exporter of combat vehicles and a major trade hub throughout the Succession Wars. People might not have gotten many visas to land on Terra proper, but it wasn't a walled off, sacred garden. It's standard jump points were major, neutral trade zones.