First thing is do some research if you are selling a poster. Do not put Blues brothers movie poster nice. that is not going to sell well
Better would be Blues Brothers 1980 one sheet folded 27x41 starring etc... Poster has no pin holes tears creases be specific on all the defects
when you do that you are less likely to get the item returned of bad feed back.
Always be clear on your shipping price and method you will keep the buyer happy.
if you are not sure on item you re selling ask a expert many of them are willing to help.
The less you say on a item the less you will get if you find you havemade a mistake correct it.
Always try to look for a date on your item many items have been remade a date helps the buyer know what they are buying.
Offer a return policy if you have made a mistake that will let buyers trust you.