Basically, supply and demand. That is, how rare or common is the stamp, and how popular is it with collectors? It's not about age. Billions of stamps were printed in the 19th century, and many of them are only worth a few pennies. On the other hand, a recent stamp with a significant printing error might be worth thousands of dollars.

As with most kinds of collectibles, condition is very important in determining value. Usually (but not always) a mint (unused) stamp with gum on the back is worth more than one that has been canceled and used for mail.

A stamp whose design is well-centered with even margins is better than one that is off center or has the perforations cutting into the design.

A mint stamp that has all its gum and no sign that a hinge (a small folded piece of glassine) has been used to mount it is more valuable than a stamp that has been hinged.

A stamp that has been creased or has a spot where the paper has been thinned out (perhaps from removing a hinge) may be worth only a fraction of the value of a stamp without such defects.

You may encounter old mail with interesting stamps on it. You may be tempted to soak or clip off the stamps. Don't do that without consulting someone knowledgable. Sometimes the whole thing with its postal markings is more valuable to collectors than the stamp alone.

Although there are exceptions, in general the following is true:

• A cigar box or similar container with a pile of random stamps (soaked off or attached to little bits of paper) is unlikely to contain much of value
• A small album, often "worldwide" of the kind kids used to start collecting is unlikely to have stamps of value. These albums were designed for beginners and had spaces only for the most common stamps.
• Little glassine packets from companies such as Harris or Kenmore  have stamps that are great for beginners but not of interest to most dealers or collectors today
• Plate blocks (groups of four or six of the same stamp with a number found in the margin). These used to be popular, but are less widely collected today. Many from the 1940s or later are now worth little more than face value.
• Whole sheets of U.S. stamps, often with denominations of 3 or 4 cents look impressive, but are pretty much worthless (with a few exceptions). These stamps could theoretically be used for postage, but you'd need an awful lot of 3 cent stamps to mail a letter today!
• A number of impressive looking "collections" have been sold by telemarketers or mail order over the years. These might include lavishly illustrated envelopes with stamps depicting a theme,  "gold" replicas of U.S. stamps, etc.) These are of little interest to today's collectors and have minimal value.

These are some indications that a collection might have worthwhile value:

• A United States collection with many 19th century stamps, especially if mint
• A U.S. collection that includes higher denomination stamps in a given series
• Higher denominations of early commemoratives such as the 1869 pictorial issue, 1893 Columbian issue or 1898 Trans-Mississippi issue
• An airmail section that includes the three Graf Zepplin stamps
• First Day covers with original artwork or illustrated by certain cachet makers
• A collection of covers illustrating "postal history" or special forms of mail delivery
• An album that specializes in a single country, such as Britain or France, with many of the early spaces filled with stamps that appear to be in nice condition
• A large worldwide album or set of albums with many spaces filled. This can turn out to have mainly cheap stamps, but an experienced person can look at certain countries and see if more valuable issues are included.

Stamp catalogs such as the Scott Catalogue available at many libraries provide a wealth of useful information. (The introductory pages to Scott are an excellent introduction to the elements of the hobby). However, there are some pitfalls to using the catalog to determine values.

• Values may not keep up with current market conditions
• In practice, many stamps are sold at considerably less than catalog value, and in turn, the wholesale value (what dealers will pay) may be only a small fraction of catalog value
• Conversely, if a stamp is in truly superb condition, advanced collectors may pay more than catalog value
• Determining which catalog entry applies to your stamp can be difficult. Scott may provide several numbers for what looks like the same stamp, but differs in perforations, watermark, or subtle differences in design or printing

How can you start or expand your stamp collection?

In the old days, every city of decent size had one or often several stamp shops where you could walk in and see a variety of stamps and be helped by a (usually knowledgable) dealer. 

Today stamp shops are few and far between. If you want to shop "in person" your best bet is a stamp show like the one the East Bay Stamp Club holds each October. (Also see our listing of other upcoming stamp shows in the Bay Area). Stamp shows give you the opportunity to visit multiple dealers.

Joining your local stamp club will also give you an opportunity, to buy, sell, or swap stamps. (Many clubs including ours also have auctions). See our section "Local Stamp Clubs" on this page for more information.)

We also highly recommend joining the American Philatelic Society (www.stamps.org). They provide many resources for collectors including an online stamps store that is well managed. Another site used by many dealers is Hipstamp (www.hipstamp.com). 

Of course there is also eBay. One has to be more cautious there, though. Many eBay stamp sellers don't understand what they're selling or describe it incorrectly. Prices can range from a good deal to ridiculous. Feedback can help somewhat.

In general, especially for buying expensive items, check out the dealer's references. Are they an APS or ASDA (American Stamp Dealer Association) member?

Finally, if you're buying a stamp that is worth several hundred dollars or more, consider getting it certified to make sure it is not fake or altered. The APS offers "expertizing" services, and there are other firms such as Professional Stamp Experts (PSE). These services can also assign a grade to the stamp.