First rule of vintage watch buying: never ever buy anything from the Ukraine. NOT ONCE have I been asked about a legitimate watch being sold from there. It is the frankenwatch capital of the world.
The next place to avoid is South America. A great many watches are shipped to S.Am. in stainless steel cases by the manufacturer only to have the dial, hands and movement installed into a locally made gold case (and they usually aren't completely gold either, they're built around a brass ring). Still nice watches if you only worry about wearing them, but not valuable to a collector. Look out for redials that the sellers attempt to pass off as original too.
Vietnam is rising in the franken/fake world. The fake Omega vintage Constellations coming out of that area are tough to spot without a good working knowledge of authentic markings and fonts.