Hi Juli, this is a great question and important subject for new collectors. So many who are new to collecting mistakenly prioritize the excitement and fun of collecting ahead of doing their research and end up losing money on reprints and faked posters as a consequence. They vent their frustration by blaming the seller when in truth, their lack of research, is their own doing. So you're really wise in having the discipline to do your research on sellers FIRST!
If you like inexpensive new double sided posters for movies from the 90s to 2010 Dale Dilts is the Mohamed Ali of ebay. He is the undisputed heavy weight champ for best prices and quality of new movie posters. His eBay store is here:
MoviePosters4u