The posters one could (and in some cases still can) buy at the box office are the official movie posters, with Eirin mark.
Sometimes distributors would print special give away posters without Eirin mark, but I have only seen a handful of those.
There aren't many reprints in Japan. The ones that are dangerous are the Toho and Daiei kaiju movies because they are official reprints by the studios themselves, and unless you know what you are looking for, it can be difficult to spot them.
But bear in mind that not all offical posters have Eirin marks. Very limited releases often do not, for example arthouse movies or the 2007 rerelease of THE TEXAS CHAINSAW MASSACRE.
Armin