I must admit, I know nothing about vintage watches but I did a search online and found these two places that have second hand Seiko watches for you (it seems more places are online only). I have no idea if they have good deals, but they both say they have Golden Week sales on.
http://www.rodeodrive.co.jp/contents3/shopinfo.aspx (an auto translation switch at top)
Most convenient store seems to be the Shinjuku branch, although they have lots of branches in Yokohama.
http://www.rasin.co.jp/ This place looks pretty flash so not sure if they'll have many deals. Here's a map URL as their site probably won't translate very well -> https://goo.gl/maps/GVs69jxJnL42 (closest station is Shimbashi)
Hope that helps.