Not a big fan of watches since I have a watch on my phone, but there are three places I've been to that sell lots of watches. Not sure if they would have any Seiko's, but the watches were nice regardless...
1) Wangfujing (yeah, I know, tourist trap, but as I said nice watches). If you exit the subway on the "south" side and head north, you'll notice a small stairwell to the right (it should be in the southern half of the walking street). I don't remember seeing the name of the place, but when you go up the stairs, you'll find yourself in a watch store...lots of watches there. Some quit expensive, but some good ones.
2) New Silk Street Mall had some watch stores as well, although I don't remember on which floor.
3) HongQiao Pearl Market also had some stalls selling watches (although it's more of a jewellery mall)
Not sure if any of those help, but you never know what gems you'll find.
