Very nice pot!  Ebay you say?  Yorozuen?  If so you have no need to worry about fakes, theyre some of the most reputable dealers in japan, and a very nice family too!
There are a lot of fakes coming out of China, but with a good eye it's easy to tell the difference.  There are a bunch of ways makers are patinating pots, and they all leave signs.  CessPool aged pots will have Patina that is uniform, real patina never is.  Washes and artificial coloring agents applied before or after firing leave brush marks, or if sprayed on it will be a negative image of the way patina forms naturally.
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I've read that burying pots in soil or compost for a period of time, maybe 6-12 months, will safely remove water marks without affecting patina.  I've never tried this, though .

Peter Krebs reccomends soaking in Coka Cola for a couple of days and then toothbrushing with clean water.  Says it works like a charm.  Haven't tried the Method yet but there's one or two pots I need to clean this winter.
The best method is prevention!  Rubbing the pots in use down with oil(many varieties will work, even veggie!)
Every week or two will prevent limescale and improve patina!  Win win.  I have pretty hard water and used to have a real problem with buildup, but no longer!  Plus you spend a little More time with trees you may not be focused on at the moment, catch problems earlier and tweak more often.
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Thanks for the info about pot cleaning, I have some trouble with calcium that vinegar has not helped with at all.  Will try the soda solution first.  I'm hesitant to try lime away, this sounds safer.  And good to know that it's preventable!

Bury it in the ground or under pine mulch for the winter...

Very nice.