As Velorum has already said, Zenitar is probably the best Vostok seller on ebay. Meranom.com and boris_gvb (ebay) are also well respected. There is one UK seller
http://www.russia4u.co.uk but his selection is pretty poor and prices aren't great. No idea how good he is as all the new Vostoks I've ever bought have been from Russia.
Personally I've never had a duff brand new Vostok but it does happen occasionally. Vostok quality control still isn't on par with the Swiss but the sellers above are known to sort any problems out without any quibbling.
edit: Oh yeah - while the watches are excellent, the straps and bracelets are garbage. Especially the bracelets!