First off, it allows people who generally have no idea what kind of RP they should be doing to get a ship that's already fully repped, it takes the work out of building the character and, in my opinion, promotes more lolwutting idiots on the server.
Secondly, most of these ship sales do not involve any kind of RP, just "I'm selling my military battleship to the highest bidder, lololol, buy eet nao!" - if you're selling any sort of military vessel , it needs to be RP'd out as having the vessel transferred to a new commander within the same faction.
Lastly, it ends up creating situations like we see with ReVan's order gunboat - which is not order tagged or ID'd, and is completely hostile to the order while friendly towards it's allies. That alone is sanctionable, but it causes a lot of problems for the faction the ship used to belong to - the order cant have random people around sirius using our technology, it's forbidden for a reason.
If a faction gives you permission to use one of their ships on an unaffiliated character, that ship stays yours or it gets scrapped, you dont go selling high grade technology to random civilians, or worse... that faction's enemies.
I dont think ship sales should have been allowed in the first place, and I wish the admins would start sanctioning anyone who does try to sell a ship.