If you are a former stamp collector, or a current one hoping to sell off a part of your collection, you have several options for where you can sell your stamps. Factors will include what sort of stamps you are trying to sell, how many stamps you hope to sell and what kind of money you are hoping to get for them.
The popular Internet auction site eBay is a great starting point for selling nearly anything that isn't big and bulky. You can use eBay to post stamps for sale with a fixed price, or you can attempt to auction off individual rare stamps or entire collections, even including a reserve price set for the minimum amount of money you will accept for the stamps. The tradeoff on using eBay is that there is a fee for posting ads, and eBay takes a percentage of some sales.
Craigslist.com, an Internet classified ad service, also charges for some listings, but in some locales, posting items for sale is free. The tradeoff is that you may be limited to buyers in your geographical region, given that there are individual Craigslist sites for each metropolitan area. In addition, some metropolitan area sites on Craigslist are more highly trafficked than others.
Join a club or membership organization for stamp collectors; this can be the easiest way to connect with fellow stamp enthusiasts and to both buy and sell items for or from your collection. Depending on the stamp club you join, you may be able to network with other stamp collectors online or in regional gatherings.
Visit a brick-and-mortar auction house if you intend to sell a large number of valuable stamps. Your collection will probably need to have a minimum value of $100 or even $1,000 for a formal auction house to be interested, although there is no minimum value required to sell on eBay.
Search for specialty stamp selling websites that will be able to connect you with appropriate buyers. Numerous such sites are available; simply type "sell stamps" into a search engine to pull up a selection of options.