How do I get the barrel of my fountain pen when it's stuck from dried ink
Prepare a jewelry cleaning kit...

moisten separate squares of paper towel in common solvents and put each in a separate  plastic bag.  make one each for clean water, water with dish-soap, rubbing alcohol, acetone (nail polish remover) and maybe... dilute ammonia and dilute bleach (but make sure these last two towels never touch each other or follow directly each other when used).  Label each plastic bag clearly with a black sharpy.  take some dry paper towels as well.

The gold nib is probably your best testing area as you know exactly what it is made of.  gold is extremely non-reactive so exposing it to water, dish soap, alcohol or acetone is completely safe.  ammonia can dull the shine on gold and bleach can oxidize its outer surface turning it black.  either the dull or the black can be buffed away with a dry paper towel, repairing this minor damage.  (once again a safety note about bleach and ammonia.  do not let them touch each other as they give off a poisonous gas when they combine).

So with your cleaning kit ready, go to the current owner of the pen and ask to run a quick cleaning test.  If the owner doesn't agree, he may know something about the unwanted ink that he doesn't want you to learn.  In which case, you should probably go find another pen.  It the owner does agree,...

Rub the dried ink on the nib across a dry piece of paper towel.  If an ink line forms on the towel, this is your base line. A solvent works if it draws a darker line than the dry paper towel.  

Repeatedly drag a dried ink spot on the nib across each solvent's paper towel. Between each solvent test, drag the ink spot across the clean water towel to avoid mixing solvents. Once you find a solvent which produces a dark ink colored line, the test is over.  The dried ink can be removed with that solvent, so the pen is safe to purchase.

If none of the solvents produce a dark line, then the "ink" will require stronger cleansers than can safely be used around the soft gold and plastic.