Prices on Oceanis 390''s tend to vary all over the place. The Oceanis 390s were not exactly robust boats. Many were in the charter trade. Used hard or used in the charter trade they could be pretty beat. There were also an extremely wide range of gear and equipment that were options on these boats. $57K sounds like a fairly cheap price for one that has had light use and has been in northern waters but may not be a great deal for one that has been ridden hard and put away wet so to speak.
If you think about the possible problems that can exist on one of these boats if it has been used hard then perhaps this is not such a great deal.
(∑ Sails, chainplates, mast step and associated suporting structure, standing and running rigging
that are beyond their useful lifespan,
∑ an engine that is in need of rebuild or replacement,
∑ worn out deck, galley, and head hardware,
∑ worn out upholstery,
∑ Out of date safety gear
∑ electronics that are non operational, or in need of updating,
∑ electrical and plumbing systems that need repairs
, upgrades to modern standards or replacement.
∑ Blister, hull deck joint or deck coring problems
. Rudder and rudder post problems
∑ Keel bolt replacement (some Beneteaus have galvanized steel keel bolts which can be near the end of thier life.
∑ And perhaps a whole range of aesthetic issues. etc)