I just wondered about this, since I found it an amusing mechanic. Perhaps it's intentional? I don't really know.
I have no problem with it if so, but thought I better mention it here in case.
(Probably not a bug...)
I have discovered that if I sell a load of gear to an NPC in order to buy all his spells etc off him... I can then proceed to mug him, kill him and steal back all my stuff again! Heh!
Rinse and repeat for all other NPCs that have any spells you might want to complete your sets.
10/10 Would be a dirty, griefing pirate again!