Obligation
As part of their Obligation, each group starts the game with a starship.
"Character Creation", Edge of the Empire p. 99
Obligation is, loosely put, the reason why your characters are going out to have dangerous space adventures instead of opening up a droid repair shop. A duty they follow, a debt they owe, a past they're trying to run from. As the plot demands it, or through their own choices, characters can acquire additional Obligation during the course of play. Most characters will start play with only one type of Obligation, but nothing stops characters from acquiring multiple Obligations of different types.
The reason you can't just pawn your initial starship is that it's not yours to pawn. It's there to answer someone else's desires, as expressed in your starting Obligations.
If your players express an interest in hijacking and selling off other starships, you can tell them up front that if they can actually manage to board one, subdue any relevant crew without causing major internal damage, slice the controls, and slip whatever civilized net might try to stop someone from taking a tremendously valuable piece of stolen property into space, they're not going to be able to get any actual credits out of one without also taking a commensurate amount of Obligation.
Because honestly, at some point in that whole process there's an Obligation-worthy event, like maybe the ship's original owners putting a bounty out on them, or owing a favor to the black-market shipyard for fuzzing the original chain of ownership so the ship could get sold off in the first place.
Now, I say "a commensurate amount of Obligation", but I should also emphasize that when you leave character creation there's no longer a fixed mathematical relationship between Obligation and credits or XP: "The GM must also determine the value of the Obligation. There is no direct correlation between Obligation and the value of credits or specific services." (SW:EotE p.310) It's just probably going to take less Obligation to cash in on a boosted light freighter than if you somehow heisted a corvette on your own initiative.
This isn't a cheap trick you use to pull the rug out from under the players, it's making use of a game mechanic you're supposed to use in scenarios where players want more resources - though usually it's by giving them a physical thing they need when they don't have the credits or can't find an appropriate place to spend them.