I agree that Posteritati's prices are off the wall but so are the prices of the other few remaining brick and mortar establishments in NYC, Chisholm Larsson and Jerry Ohlinger. But consider this, the monthly rent for any storefront in NYC is astronomical so I personally cut them a little slack and admire them for keeping their lights on for walk in customers. I did not say I would purchase from them, though.
As to reprints I don't really see the harm. I know we are all purists here but lets say you really, r e a l l y want a particularly expensive poster that you know you will never be able to afford. Casablanca, a Fistful Of Dollars advance, Citizen Kane 22x28, whatever. I would not be adverse to buying a well printed repro of it, as long you know up front it's a repro and the seller is also honest about it and tell you it's a fake. All good. It fills a hole in the collection. It's when certain people knock off things like a Dracula 1-sheet, fold it up, wear it out, splash mud on it, then get it linenbacked for the express purpose of ripping people off. That's just fucking wrong. The same goes for flat out bootlegged Star Wars posters and the notorious minty white inserts. Those should all be run through the trusty crosscut shredder and be turned into confetti, which is about how much they are worth.