Ebay Observations
I do enjoy ebay as a resource for what other sites offer as well as looking for items for myself but I have noticed a few things that you might want to consider.
It seems there isn’t any mediator that would police how artworks are listed or catagorized. For instance, if you narrow your querry to original posters there seems to be quite a lot of reproduction posters listed. Or if you choose, “signed” there will be quite a few listed that have a signature as part of the print but not “signed by the artist”. In the prints department I seem to see many of the same type of listing issues. My recomendation would be to make sure to check with the seller whether the print or poster is an original image and if it states it is signed verify whether it is “plate signed” or “hand signed”. I tend to think some of the people posting items may take their cue from other listings and may be naive about some of the terminolgy regarding fine art or collectable posters so the more you know the better chance to get what you pay for or perhaps find a great piece at a gerat price.