Instructions
Identify the origins of the postcard and the age. There are sites available on the web to help with your research or you can visit your local library for books to help you determine where and when the postcard was produced.
Assess the quality and determine a grade. Standard grades range from "mint" condition to "fair" condition with several in between.
Visit stores that specialize in the sale of vintage books and postcards and talk to the staff about the postcards you want to sell. They can help you gage how much of a market there is for the particular cards you have and help you determine a value.
List vintage postcards for sale using on-line auction sites. Selling to a worldwide market will increase the chances of a sale. Just make sure you understand listing and final value fees to ensure you maximize your profits.
Take your postcards to a consignment shop that will sell them for you. They will generally keep a percentage of the selling price as a fee, but there aren't generally any fees involved if the items don't sell.