There should be no tone spots _ marks of a brown, rusty appearance _ on either side of the stamp.
Hinge remnants, where the stamp has torn away rather than letting go of the stamp, should be clear and clean and not thick and dirty.
The perforation teeth surrounding the stamp should look nice, not be too short or torn.
For cancelled stamps, the same guidelines should be used but note the postmark. The stamp should have a clear cancellation mark but not too heavy, as that could affect its price.
In other words, the price of a stamp in good condition can be higher than the catalogue or even market price.
rint colour should be fresh, not faded.