The value of a stamp is determined by several factors. First of all, it is rareness. According to the law of demand and supply, if there is a lot of people want a stamp, the value of it goes up a great deal. And the harder a stamp is found on the market, the more valuable it is.  In addition, the value is also determined by condition and grade.  Grade is about how well the stamp’s design is centered between the perforations, both horizontally and vertically. Condition is about the physical characteristics of the stamp’s paper, including ink, color, and gum in the case of mint stamps. 

 

Superb Grade
•  Perfect in all respects. The finest quality.
•  A rare grade.

Extremely Fine Grade
•  Close to perfect grade.
•  Design is very well-centered. Margins are even all around.
•  Condition is rich and printing color is bright.

Very Fine Grade:
•  Clean margins but not slightly off-center, or may be off-center either horizontally or vertically but not both.
•  Imperforates have three normal sized margins. .
•  Condition is rich and printing color is bright.

Fine Grade
•  Design is "noticeably" off-center both horizontally and vertically.
•  Imperforates have thin margins.
•  Condition is no faults.

Average Grade
•  Design is off-center and perforations  may cut into the design.
•  No tears or thin spots.
•  Condition has slight faults.

Poor:
•  Design is off center and the perforations cut far into the design.
•  Cancellation is thick and heavy, smeared, blurred.
•  Generally not suitable for a collection.

 

Here are some cool stamps for sale on eBay.