Here are some useful links for dating your Zippo.
Moonlight Zippo (Dating Regular)
Outside Hinge Laboratory OKZIPPO
The Earliest Zippos 1933 to 1936
The History of Zippo Lighters, Zippo Collectable, Zippo Lighter, Lightermall.com
The passionate wick (town and country)
Zippo collection (Paul's Zippo collection)
New to collecting?
Buyer beware, do not believe the seller (unless a respected collector.) I have seen many examples of sellers falsely claiming a Zippo's manufactured date on well known commerce sites. They justify their false claims saying they are no expert! Selling a Zippo as thirties for example(very rare and worth likely four figures,) when it is only worth two figures! It is a practise I wish the commerce sites would endeavor to prevent via seller guidelines, it is a false claim after all! To them I say here, protect your buyers, trust is important. If the seller is no expert, who are they to date it as the most expensive?
Feeling rich? :-) here is an actual thirties Zippo;