What is my stamp worth?
This is another common question I get asked, and it's not a question I can easily answer. I'll try to give you some general answers, at least for U.S. stamps:
There are exceptions to all of these guidelines, however. There are variations of common stamps that are worth considerably more to certain collectors. There are postmarks that can make a common stamp more valuable. And there are always errors - mis-perforations, missing colors, upside-down vignettes, and so on, which are usually much more valuable than the normal version of the stamp.
So how do you determine the value of your stamp?
Your first step should be to go to a decently-sized library and look in their reference section for a copy of Scott Specialized Catalog of United States Stamps. Here, you can find a good estimate of what each stamp is worth. For the more common stamps, it's pretty easy to find the stamp that you have, using their denomination or subject guides. You can also use my denomination guides and your browser's Search function as the first step in doing this. Use my guides to find the Scott number and verify that my copy of the stamp looks like your copy of the stamp. Of course, if I don't have that stamp, my site won't help you... Once you know the number, it's easy to use the Scott catalog to get the value of the stamp. The Scott Catalog will also tell you if there is more than one variation of your stamp, and if so, how much it is worth.
In some cases, there may be many different versions of the same stamp. For instance, these two stamps are two different issues, with two different values:
For some issues, you may have no practical alternative than to take your stamp to a dealer or experienced collector and ask for help.