This list just scratches the surface. There are many stamp and postal history dealers who are listed on some of the other sites that our links take you to. We are not endorsing any particular site or dealer.
American Philatelic Society
Non-members can buy, and members can buy and sell through StampStore
U. S. Postal Service Store for Current U.S. Stamps
Add current U.S. stamps to your collection at face value
This online store is a great place to search for stamps and covers
Another online source to purchase (or sell) stamps from around the world
A European website that works very much like ebay, with millions of stamps for sale
An online market for the sale, purchase, and trading of stamps.
Consignment eBay sales specializing in rare stamps, coins, paper money, books, antiques, and collectibles.
Stamp Market Index
Searchable database of all eBay stamp sales since 2004.
Online retailer specializing in philatelic books.
Online retailer specializing in United States & Worldwide Philatelic Literature
List of Stamp Dealers
This is a way to search for stamp dealers who are also APS members and who subscribe to the APS Code of Ethics
Another Stamp Dealer List
This includes a list of American Stamp Dealers Association members, another group that requires members to adhere to their Code of Ethics. They also publish a magazine with articles of interest to stamp collectors, not just dealers