I hate to be the one to say it but i think you got scammed buddy. That poster looks like it belongs in an episode of Yo Gabba Gabba and could have easily been put together with clipart. That is nowhere near "art" and for $150 i can buy a screen printing kit tons of paper and make 1000 of them with nothing more than photoshop.

Tell ya what, when nobody else makes an offer to buy it, I'll give you $5.00usd for it. I can hang it next to that Teletubbies poster i have always wanted.