movieposter.com has inflated prices to put it mildly on their older stuff, so I would recommend to be patient. Regardless of what all the "ebay sucks" posts say, it still is the number one place to get decent prices on posters, period.
Like Mel stated, most reprints are give aways once you have them in hand do to contrasting paper makeup or how bad the printing is. The ones that will get you are when original plates were used or someone had access to the high dollar printers to run off something and most of these posters are known in the community.
Usually if you look at everything a seller has or research their history, that too will be a good hint as too what they have going on. For example, they have all 90's posters, but are advertising several copies of a rolled early 80's poster as original, be wary. Things like that, were there is a contrast in what the bulk of their inventory is. If they accept returns, no big deal, role the dice and see what happens.
