I did bid a FA 31mm in japan also, if you buy it via kakaku, and the seller has good reputation the lens would be in a good condition too, if you can check at store it's even better. take your camera with you, and shoot some photos to check the focus, put the cap on the lens and shoot (total black on the screen) to check if there is any dust/fungus. if you have any friend who knows about lens, ask him/her to come with you.
500$ is a steal for this lens (in good condition), you won't see this price ever again !
I was new to photography too but I instantly see the difference when I shoot with fa 31mm
life is too short, don't wait if you already see an opportunity, take it ! you'll love it
PS : check the
review of this lens and see if you like it's rendering as we all do ^^