So, Jamie Boran knows his stuff and I'm sure if it was from his collection, he believed it to be authontic. That does not mean that he was not tricked. MJP nailed it. I really have issue too with the markers looking so perfect after all these years. Also, the paper looks to be some kind of posterboard, which would most likely have had a high acid content, yet it seems to be bright white. Even if is was kept in a dry and dark place it seems that it would have aged more. My guess is that this is not nearly as old as it is advertised. I would be surprised if it is even as old as 1995. Also, as Jordan pointed out, the provenance does not make any sense. Why did Buk call Sandy "Neeli"? He would certainly have known them both very well in 1969. Also, I can't tell who they are saying tore the poster down. Finally, the poster looks more like an art project than a poster announcing a reading.
Avgent is a reputable seller, but the Provenance is lame at best. Maybe Neeli can shed some light on it although I can not imagine that he would remember a poster from 45 years ago.
Even if it was 100% real, it would not be worth anywhere near the asking price. As there are so many questions, I cannot imagine that anyone but someone with too much money and too little brains would pay the price...
If someone offered it to me for $100, I'd likely pass.