At the end of the day, and taking into account things like strength of the market, trends, auction or dealer, strength of auction etc, all which will affect its final sales result, if a never before offered poster, without any previous auction history, sells for $10,000.00 (for example), then doesnt it make sense that this is its value?
Until this poster might go on the market at some point in the future, (or another copy was found and auctioned or sold), I would think that realized price, would serve as a starting point for that particular poster's auction history.
I would guess this is how auction histories are started for anything, posters or otherwise. They all start with the first "one" found and offered or sold.