One of the most important factors when it comes to determining the value of a stamp is its "grade". This method of rating the overall quality of the stamp is usually carried out by an expert (who will then provide a certificate), but you can get a good idea of your stamp's grade by following a few basic steps.
It should be noted that the grade of a stamp is separate from its condition, yet both rely heavily on each other. For example, if you have a stamp that has a high grade but is in poor condition, its value is still likely to be low - and vice versa.
Grading is determined largely by a combination of three factors: soundness, centring and eye appeal.
Soundness is the term used to describe presence or absence of defects in the stamp, such as gum skips, pinholes or colour fading. The different levels of soundness span from Faultless to Severe Fault and there are detailed guides online that will help you to determine the level that your stamp is at.
Centring is the position of the design on the paper. Obviously, a stamp where the design cuts into the perforations is less aesthetically pleasing than an example that is well centred with four equal margins, and this plays a large role in its value.
Grading the centring of a stamp is a more complicated business than soundness, as different printing techniques have been used over the years and have gradually become more precise. Therefore, what may be a good grading for an early stamp could be a poor result for a more modern issue. This mainly lies in the addition of perforations, and while a sheet may have been printed correctly, the perforations can be misaligned, leading to the appearance of poor centring.
However, if your knowledge of the stamps within your collecting area is sufficiently advanced, you will soon recognise what issues are poorly centred and will be able to determine the grading accordingly. Levels range from Gem to Good.
And so to the third aspect: eye appeal. This is simply the attractiveness of a stamp and depends on its colour, the quality of any cancellations, impression and freshness.
As previously mentioned, this is just a rough guide to grading and, depending on your country and which issue you collect, there will be varying opinions. For stamps over a certain value, you will need a certificate from a professional authenticator before selling - read our guide on choosing an authenticator