I have found that most dealer prices are negotiable.
I wouldn't be too quick to pay full price before any dickering has been done.
Get the dealer as hungry for the potential sale as you are for the potential purchase.
Communicate with the seller.
Ask lots of questions about the poster.
Request large scans because a sale is imminent.
Closely examine the condition of the poster and highlight any and all defects.
Use these flaws to drop the asking price down one, two...maybe three notches or more.
Maybe you'll get that poster for a lot less than it is advertised.
Never swing at the first pitch.
And... be patient grasshopper.
I had a beautiful poster on my Ebay watch list for over one year.
Only one I could find anywhere on the internet.
A long time ago, I made the seller an offer which he politely refused.
I really wanted that poster but I could not justify spending the money.
It sat for months and months on my watch list. No takers.
Just as I was about to break down and pay his full price, another identical poster appeared at 1/2 the price (shipping included) and in better, unused condition.