Buying a ship can be, at least early on, one of the biggest accomplishments a party can achieve. You get to start with one, but once you have enough resources to actually go get one of your choosing, that's bragging territory, in the lower echelons of the Expanse, anyway. What about that rare day, though, when you find a ship, and it's in good condition, but you truly just don't want it? It might be hard to justify not wanting another ship, considering how useful it could be, even in the background, being flown by your organization, but if we just say you want to sell it, how much PF might you be able to get, on average? Say you found a planet that, until recently, was socked in a warp storm, and above it, there had been a shipyard/dock. In the dock ring, you find one or more functional ship hulls, maybe not pimped out, but none the worse for wear. The planet below is dead, and the ships weren't military assets, so the Navy can't just walk in and say "thanks for recovering our ships, which we are now taking." If you could legitly keep them, but don't want to, what could they be worth? Buying one is big, nut your PF doesn't decrease, whether you buy one or ten, unless you actually spend PF, in order to ensure your success. That means a 50 SP ship can't be worth 50 PF, so how many is it liely to be worth, if you were of a mind to sell it?
Sorry for the long read, and the repeating myself frequently. I just wanted to get the point home, eventually. Thanks much.