Condition is the most important factor in determining the price and fair value of a stamp. Markets are obsessed with quality. When buying a stamp for investment, few compromises are acceptable unless a stamp is unique or so rare that few investment quality issues exist. Even then, keep in mind that catalog prices are generally set for Fine to Very Fine (F/VF in stamp speak) copies with average centering and, for mint stamps, with gum and hinge marks. Stamps up to the 1920s which are mint never hinged (MNH or **) often carry a premium of 200% or more of catalog simply because prior to that time there was no clean way mount the stamps in an album without a hinge of some sort. The exceptions here are for countries were unsold stamp stocks were sold to dealers as remnants and the large quantities meant many stamps were not sold until hingeless albums and plastic mounts came about. Well centered, fresh looking copies and jumbo margins for stamps from the 19th century also can carry premiums of 150% to 500% based on their rarity and popularity. Pricing for 19th Century stamps cataloging over $100 can vary widely because printing and engraving were still a primitive art in most nations. The majority of 19th Century stamps contain faults. The front section of most stamp catalogs will feature detailed explanations and illustrations for stamp centering and gum.
You may hear the term investment grade applied to a stamp as if this mitigated any need for skepticism. Assume the worst, and you won't miss defects your buyer won't. Defects are many and they carry a heavy penalty in pricing. Defects includes:
The importance of a stamp's condition is such that professional help is available to assure that a stamp's representation is accurate. The American Philatelic Association offers a service at reasonable cost which accurately describes a stamps condition as well as whether it is genuine in all respects. Such a certificate makes a stamp much more saleable to a distant buyer or an auction house. In fact, when buying at an auction, obtaining a certificate after purchase, but before payment is common practice for higher priced items. It is an acquisition cost well worth considering for used stamps and stamps with overprints or other special features subject to forgery or falsification.