Today, almost all film posters are available in reprint form from various outlets including eBay, but these reprints are almost always very easy to spot as the print quality is poor, they are notably undersized and the paper they are printed on is incorrect.
Many film posters were also officially licensed by the studio or distributor for mass reproduction, but these reprints are always easy to differentiate through signs such as the size, quality of print detail and addition of copyright notices and other markings not found on the cinema printings.
For example, regarding Studio Ghibli’s posters, there is a set of 22 reprints of the original movie posters that was authorized by the studio itself and sold at exhibitions. Here you can find one of those. As far as I’m aware, these are not original movie posters.
Over the decades there have been film posters that were more desirable than others and there are instances of these being copied (forged) and reprinted. Most of the time these fakes are fairly straightforward to spot, although there a number of titles that have had very high quality copies made and they can fool both dealers and collectors alike.
The reprints/fakes issue is something that any collector should be aware of, particularly since there are so few guides available online that detail these fakes. Some that exist include: (i) Learn About Movie Posters has a guide to fake posters; (ii) In CineMasterpieces you can also find some articles about this topic.