If you plan to sell your posters locally or by mail or whatever, have a word with some local print bureaus and the like, get a quote for limited print runs and possibly sample media.
If you want to do it online, you can get CafePress or other online shophosters to do all the legwork - upload your graphics, and choose what you want people to be able to buy - customers'll then be able to order direct, it'll be processed, printed on demand, and you'll get the larger chunk of the takings.
Simple.
Can't really recommend anyone though - never actually seen any prints from online sources, and I'm sure we won't be in the same city to talk about bureaus...