Determining value of anything the condition is the most important point to consider.
Many professional appraisers use eBay to determine value.
Checking present and past eBay auctions have determined your poster is a rare poster.
Checking for vintage posters on eBay they have brought under $10 to over $70.
Your poster in mint condition could be worth more than $200, and in poor condition may not be worth the money to pay a professional appraiser to appraise it.
I would suggest putting a value on the poster, that you feel is more than the actual value, and then maybe take less if you have an offer close to this amount.
Electr&onics Tech
30 Plus years Electronic Tech - 50 + Years Handy Man - 50+ years Gardner - Researcher - Home Owner -