For what it's worth, you can buy with confidence from Bruce, Heritage, or Dale Dilts. If they say a poster is legit, it's legit. Also, I agree with eatbrie, stay away from Loce. He has no problem passing off fakes as the real deal.
The only surefire way to tell if a poster you want is real or fake, barring a very high res photo, is to see the poster in person. Usually, the small print like the credits are a dead giveaway. If they look fuzzy and not sharp and clear, it's a fake.