Pbartender said:
1. Supply vs. Demand - How many people are there around who can afford to buy a spelljamming ship and its helm? How many of those people have the interest to buy one? How many of them have the need to buy one? How many of those are looking for a spelljammer ship that fits the description of what the PCs are trying to sell? Most of the people who have the resources, need and willingness to buy a ship will already have one. It's hard to sell something that no one wants to buy.
It is, however, vastly unlikely that in a universe of crystal spheres with multiple inhabited worlds, that has commerce and individually-owned vessels, that buyers could not be found. Especially if you're willing to take trade in something else. Of course, the vessel may not fetch as much as you'd really hope, kind of like a used car.
If spelljamming vessels are common, then there has to be a market in them (but see used cars analogy again). That's true even if they're rare; someone will want it, you'll just have to work harder to sell it off. Certainly, the Great Powers were perfectly happy to press captured enemy vessels into service* (several British men'o'war at Trafalgar were formerly French, and vice versa).
The stumbling block that I find *much* more likely, you do touch upon. The PCs may not have enough crew to safely get the prize back into a friendly port. Even then, they may not get enough money to make it worth their while. If spelljammers're common enough that they're not worth terribly much, then several months of crew pay and rations, with the added risk of losing it and your trained crew (and possibly a party member) irretrievably, may wind up being more than you're willing to pay to sell the ship you captured. It's likely to just be easier to grab their cargo and run. (It's like that pile of 100,000 copper pieces...it's just not worth the effort to haul out)
Of course, this leads to some interesting decisions. If you're not going to try and keep the ship, then you can let the enemy ship go, free to give your descriptions to their allies who may then decide to hunt you down. Or, you can slaughter everyone on board, but this may provide some moral dilemmas (not always, but sometimes) and leave it to drift as a hazard to navigation. Or you can destroy it completely.
Brad
* - Note that these were taken as part of military actions, but it illustrates that second- and third-hand ship ownership is possible.