Here are some questions to ask your self when buying a poster:

1. What condition is the poster in? Is it new, mint condition, have any holes or tears?
2. What kind of feedback does the seller have? Feedback is important of course, but it's important to see if the seller has any negative feedback regarding honest transactions. If the seller has sold a poster inaccurately don't buy from them.
3. Is there a shipping discount if you buy more than one?
4. How soon do  you need the poster and how fast does the seller ship them out once payment is recieved?

It's important that posters's are shipped in a sturdy tube so no damage comes to the poster during shipment.  If the seller does not specify that...send them a "ask seller a question" e-mail to make sure they do.