Knowing stamp condition is important for valuing your stamps and for either buying or selling stamps.
Be very careful when evaluating your stamps' value or when buying stamps from someone else. MAKE SURE to examine them for faults. Even small faults can have a huge impact on the value of a particular stamp.
99.9% of all stamp sellers are honest, and they fully describe all their material being sold. The few that aren't honest though, give everyone a bad name. It is wise for one to carefully examine any stamps being bought, to make sure they are worth the price being paid.
Faults can be short or missing perforations, tears, creases, thin spots, staining or discoloring, a heavy cancellation that completely obscures the stamp design, etc.
A damaged modern stamp or a damaged stamp having very low catalog value is "worthless" and should not be collected. A nearby trash can would be a great place for such a stamp.
Classic stamps, through the issues of the early 20th Century,
with such faults, depending on their severity, are usually sold for
considerably less than the "retail" value (about 10% - 20%) for the same
stamp without faults. If the faults are severe, the stamps are
somtimes referred to as "seconds" or "fillers".
Some stamps that were issued with large perforations (early Finland), are printed on poor quality papers (many 1850's issues), or are printed on very thin papers (early Hawaii), are sometimes expected to have minor faults, such as an occasional short perforation or maybe a shallow thin area in the paper. These do sell for a little more, as a significant number of the ones on the market are expected to have "minor" faults. It depends on the particular issue. The major catalogs usually note this, either in their stamp condition sections, or in comments following the particular catalog listings for the stamps.
In the evaluation of RARE and HIGH PRICED classic issues, not all faultiness is considered "bad" or "undesirable".
My Switzerland Scott #1 (above) is certified as being expertly repaired, and it still cost me a three-figure price! With classic Switzerland, as an example, almost all the available Cantonal issues on the market, that are offered at discount prices, are assumed to be "repaired". Ones that are "sound" and superb in quality, appearance, and centering usually bring astonishing prices, especially at auctions, where the competition for them can be intense.
a Hawaii Scott #1, at auction, even though it may be extensively
repaired, may go for over the full catalog price, as the demand far
outweighs the availability.
Many times, collections and accumulations that have been improperly stored, such as in garages or basements, will have some or all of the gummed stamps stuck together. This type of stamp condition can be the case with old stock books, albums, and with glassine envelopes that are stored in a careless manner.
Unfortunately, soaking is the only way to really get them apart, without causing further damage to the stamps. Mint 19th Century stamps may be preferred without gum, as some of the old adhesives can destroy the stamps. Mint 20th Century stamps without gum are worth a very small percentage of the same stamps with gum, but they are the REAL stamps, and they will still look nice in your collection, until you can replace them with full gum examples.
old method is to steam them .... hold them over a tea kettle or pot of
boiling water, until they loosen and can be pulled apart. This,
however, can result in thinning or tearing of the stamp. Remember, a
modern mint stamp with no gum can have a “little” value. A damaged
modern stamp is pretty much worthless.
The key element in determining stamp condition and detecting faults is the amount of STUDY put into the stamp collecting effort and into your specialty! One should read the grading information in the catalog thoroughly, and one should familiarize themselves with the details of ALL the catalog listings for the particular area that is being collected. KNOWLEDGE is the most important necessity for any collecting endeavor.