Interspace doesn't sell the parts.
The reason the Parts are for sale there is because Ageira uses the IC bases to store the parts for repairs within the respective houses, rather than bringing them all the way from Pueblo.
Interspace can move them only because of a contract between the Player Factions.
As for monopoly, I see where you're coming from Prowler, but H-Fuel is sold all over Sirius, even if it is second hand stuff. It's not an exclusive commodity, as -everyone- needs it.
G/L Parts on the other hand, are manufactured in one place and one place only. Their movement and handling is very closely watched and highly restricted. We can enforce this monopoly in game, because via the rules we demand them from lawfuls.