I agree with bernard_L -- there's not a lot of used darkroom gear available on Beaumarchais -- the only items I've seen are in Odeon Photo on the left side of Beaumarchais heading toward Place de la Republique. There will be some at Bievres, though I was specifically looking for darkroom items last year there and didn't find very much. FYI, the Bievres website says the camera fair starts at 2 PM on Saturday, but I recall last year that it actually started in morning -- so don't hesitate to get there early.