Held hostage? Just sell the damn stuff. You can always get more. With the way mission rewards have changed and you no longer have to worry about being paid in 4t of junk (or worse, trying to build up 25t of modular terminals 4t at a time) there's almost no need to stockpile anything any more. The absolute worst one is Bill Turner and his bromellite but even without the new mining system that was just a case of taking a mining ship with 50t of cargo space and hitting up an icy ring.
Except when it's mission rewards that you have to store for, say, unlocking an engineer (COUGHMARCOF*$&%INGQWENTCOUGHCOUGH sorry had some crap stuck in the back of my throat there...yes I put up with that nonsense because I wanted my G5 thrusters, damnit) and you'd much rather fly a different ship that's not cargo fitted to get things done but you already have stuff in your hold?
Or when the situation changes somehow between the moment you've bought the cargo and the moment you're ready to go. Like, say, all your friends just got on and want to wing up for some bounty hunting? (which, I might add, is objectively more exciting than hauling 700+t in a T-9?)
Or maybe when you're just tired of mining and want to go to koli Discii to farm some mats so you can build out your new Mamba...and you're still sitting on 200 limpets. And now you have to wait on the damn sell function to tick down to 0 instead of having a "sell all" feature or a way to just input "Zero."
If you can't think of multiple reasons a player would want to switch ships and NOT have to jerk around the menus to sell everything in their hold, then you and I are playing VERY different games...