I bought some filters from KEH. One was impeccable, the other was so dirty I don't know how they even rated it EX or EX+ or whatever it was, but it was fine after a good rub down. I have heard only good things about KEH, all first hand from at least a dozen or so people. I would feel safe getting more lenses for my G there. I don't know about any of the others.
But nothing beats finding it locally in a used store and getting to try it before ya buy it though.