In the few altercations I've had with ebay sellers. Ebay have only sided with me once, they 90% of the time back the seller weather right or wrong.... This is another reason I stopped buying from over seas, I was spending more on postage than posters, and any problems it was more cost to send items back.
Luckily this was a cheap poster, so just give the negative feedback, post the name of the bad seller, as you have done here, and get on with your life. These people aren't worth the hassle. Now if you were in the same country as the seller, a knock on the door, and a big bat sort most problems out, I've found. ....It's great exercise too.